This work is part of a multi-institution collaboration developing a fair, valid, and reliable research-based conceptual fluids assessment, the Fluids Conceptual Evaluation (FCE).  The FCE will utilize two-tier multiple-choice items covering both fluids statics and fluids dynamics.  Rasch analysis will be used to create different equivalent versions of the FCE, allowing instructors to give just the statics questions, just the dynamics questions, or a combination of both.  In this poster we will describe the development of student-based distractors for the FCE.  IPLS instructors interested in serving as a pilot test site should contact Project Lead Dawn Meredith.

Development of student-based distractors for the Fluids Conceptual Evaluation (FCE)

Magnetorheological fluid (MRF) is a magnetic particle reinforced fluidic material, which is mechanical properties, such as stiffness and damping, can be controlled with the application of a magnetic field.1 Smart sensor and actuator can be fabricated for adaptive vibration control systems where mechanical properties can be controlled by the application of a magnetic field. Manufacturability of MR fluid systems using the additive manufacturing (AM) process has gained great attention to enable the geometrical design of the MR fluid in an elastic component.2 In this research, MR fluid was prepared by carbonyl irons which are 6 – 10 μm in diameter with silicon oil (40% volume fraction). Thermoplastic polyurethane (TPU) with shore hardness 60A was used as an elastomeric system. A 3D printer is used to produce cylindrical TPU elastomer with grid structure using 50 % infill (Figure 1a). The MR fluid was added into the TPU elastomer, followed by adding a sealing layer with 3D printing. An external magnetic field (0 and 7 kG) was applied along the sample longitudinal axis at the time of uniaxial dynamic testing with 10% pre-strain. The TPU-MRF samples were characterized via force-displacement tests conducted at 1 Hz under strain amplitudes of 2%. The MR effect on the dynamic stiffness is defined as the ratio of the dynamic stiffness under the maximum magnetic field to the dynamic stiffness in the absence of a magnetic field. Mechanical testing results with the magnetic fields show that the MR effect on the dynamic stiffness is 1.65. (Figure 2a). The area of the force-displacement curve at 7 kG is about 6.5 times larger than the result with 0 kG, indicating a significantly large change in damping property. Frequency was varied from 1 to 10 Hz and the force-displacement curve didn’t show significant changes, indicating that the performance of the TPU-MRF system doesn’t diminish with increased frequency. (Figure 2b)

![](https://assets.underline.io/uploads/markdown_image/1/image/19adad7fc103540f67a0f0d92653914f.jpg)

Figure 1 (a) Schematic diagram of a TPU-MRF sample fabrication (b) Image of TPU grid structure, TPU sample with and without MR fluid.

![](https://assets.underline.io/uploads/markdown_image/1/image/38dc14fab1586b1c5bfaea4cfc340707.jpg)

Figure 2 (a) Force-displacement curve of TPU-MRF with 0 and 7 kG. (2% strain) (b) Force-displacement curves of TPU-MRF with a different frequency of vibration at 7 kG. (3% strain)

Encapsulations of Magnetorheological Fluids using 3D Printed Elastomeric Rectangular Cellular Structures

Two-dimensional magnets based on van der Waals materials are currently fostering great expectations for the advancement of spin-orbitronics, which aims to exploit the spin-orbit coupling in non-centrosymmetric magnetic heterostructures to enable current-driven magnetic torques and stabilize homochiral magnetic textures. Van der Waals magnets are particularly appealing for this purpose as their properties can be tuned by surface engineering at the atomic level. In this talk, I will present our investigation of the spin-orbitronics properties of selected highly promising van der Waals candidates, both in pristine and Janus configurations, from first principles and effective tight-binding models. I will first discuss the spin-orbit torque that emerges from inversion symmetry breaking in magnetic transition metal dichalcogenide monolayers [1] and demonstrate that sizable torques can be obtained, whose magnitude is controlled by the electric dipole due to the chalcogen elements. Most importantly, I will show the existence of a unique spin-orbit torque component that allows for field-free current-driven switching, of highest interest for applications. Then, I will discuss the nature of Dzyaloshinskii-Moriya interaction in these systems, and how they can stabilize magnetic skyrmions, but also magnetic bimerons depending on the magnetic anisotropy properties [2]. Finally, I will discuss the spin-orbitronics properties of Fe3Ge2Te2 and show that in spite of its high symmetry, its inherent mirror symmetry breaking enables the onset of an in-plane Dzyaloshinskii-Moriya interaction that can stabilize planar chiral textures [3].

[1] Smaili et al., arXiv:2007.07579
[2] Laref et al., arXiv:2011.07813 
[3] Laref et al., Physical Review B 102, 060402(R) (2020)


Theory of spin-orbit torque and Dzyaloshinskii-Moriya interaction in van der Walls magnets

* Corresponding Author: ozan.akdogan@eng.bau.edu.tr Magnetically actuated milli swimmers/robots, especially in biomedical field, are used in many applications such as cargo delivery and sensing applications. In this study, magnetically actuated small-scale swimmers have been developed by using Fe flakes embedded in photocurable polymeric resin. Our future aim will be to merge the developed small-scale swimmers with minimally invasive biomedical applications. The 3D printed structures were fabricated using a photocurable polymeric resin acquired from Wanhao (China). In addition to monomers and oligomers, the chemical composition of this resin includes a photo initiator that responds to a certain wavelength of UV light (405nm) to change the polymerization conformation of the resin into a solid state. Then, structures were printed by using the Anycubic Photon digital light processing (DLP) 3D printer with an exposure time of 10 seconds and a layer thickness of 0.05 mm. Different sizes and shapes of (length of the ship1, ship2 and square, 5.6 mm, 2.7 mm and 0.6mm, respectively) the magnetically actuated swimmers were 3D printed; two ellipsoid shaped swimmers with different sizes and a cubic shaped swimmer. An ellipsoid and cubic shape were chosen for its unique morphology that gives it hydrodynamic superiority over traditional spheres [1]. The combination of polymeric materials such as resins with magnetic particles expand the robot’s functionality [2]. The Fe flakes have been previously tested with functional polymer composites for extrusion-based 3D printing [3]. In this study, Fe flakes (100-200nm thickness and ~10μm lateral size) were incorporated into the photocurable resin to actuate robots in response to the magnetic field. The X-Ray Diffraction and vibrating sample magnetometer analysis of Fe flakes showed that Fe flakes have BCC crystal symmetry and soft ferromagnetic response, respectively. (Fig 1a and b). A 0.2 g of Fe flakes and 40 ml of the polymeric resin were mixed to synthesize 0.5% wt/vol concentration. The printed structures were cured by UV light exposure. The incorporated Fe flakes were visualized by a light microscope (Fig.1c, d and e). The movement characteristics of the swimmers in a magnetic field gradient were tested by a homemade electromagnetic setup with a maximum field of 336 Gauss. The 3D printed magnetic structures’ movement speed in DI water were calculated experimentally as 12.27±1.89 mm/s, 11.92±1.9mm/s and 8.29±1.5mm/s for milli ship1, milli ship 2 and sub-mm square, respectively. The theoretical swimming speed values were calculated according to the equations below; the magnetic force acting on the robots by a gradient, in which m and B are the magnetic moment and the field, respectively (3) and the drag force (Stoke’s Law), in which, η, the viscosity of DI water (8.9x10-4 Pa.s), R and v are the hydrodynamic radius and velocity of the objects, respectively (4). Fm = ▽ (m . B) [3] Fd= 6π η R v [4] Theoretical speeds calculated for milli ship1, milli ship2 and sub-mm square were 13.54 mm/s, 9.9 mm/s and 10 mm/s, respectively. The different shaped milli swimmers were actuated by electromagnetic system and tracked (Fig.2). Magneto-responsive robots movement abilities will be tested against specific obstacles with different shapes and magnetic materials as well. Acknowledgements This work was supported by BAP.2019-03.11(Bahcesehir University).

**References:**

1. Carrasco, B. et al., Hydrodynamic Properties of Rigid Particles: Comparison of Different Modeling and Computational Procedures., Biophysical Journal, 76, 3044–3057, (1999) 2.Song, S. W., Jeong, Y. & Kwon, S., Photocurable polymer nanocomposites for magnetic, optical, and biological applications., IEEE J. Sel. Top. Quantum Electron. 21, 324–335, (2015) 3.Zirhli, O. et al. Fabrication and Characterization of Fe16N2 Micro-Flake Powders and Their Extrusion-Based 3D Printing into Permanent Magnet Form., Adv. Eng. Mater. 22, (2020) 4. Pankhurst, Q. A. et al., Applications of magnetic nanoparticles in biomedicine., J. Phys. D. Appl. Phys. 36, 167-181, (2003) 5. L.D. Landau, I.Akhiezer, M.Lifshitz, Chapter XV-Viscosity, General Physics, 341-366, (1967)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/fcd27136415135330892ad5e005270ef.png)
Figure 1. a) VSM and b) XRD analysis of Fe flakes given. Light microscopy image of Fe flakes nanoparticle distribution 3D-printed structures c) Milli square. d) Milli ship. e) Texture of printed structures.

![](https://assets.underline.io/uploads/markdown_image/1/image/66e69fa67a75757fe051df0e36d11fd7.jpg)
Figure2. The swimming pathway of structures in the DI water are shown in the figure respectively. a) Square, b) Ship 1 and c) Ship 2 (0.6 mm 5.6 mm, 2.7 mm, respectively). d) The swimming trajectory snapshots of robots under the magnetic field.

 Development and characterization of magnetically actuated milli-robots by Stereolithography

Implementing magnetophoresis in microsystems inspired vivid interest in biomedical applications as it meets a need for trapping and separation methods enabling specific and continuous isolation of biological objects for further analysis. Magnetophoresis, referring to the motion of an object in a magnetic field gradient, is a highly selective and versatile sorting method and, contrary to concurrent methods such as immunoaffinity or dielectrophoresis, it doesn’t depend on the medium properties (pH, temperature, ionic strength...). Downscaling the size of the magnetic flux source considerably increases the generated magnetic field gradients generated [1], but inevitably reduces the interaction distance. Therefore integrating micrometric magnetic structures, within a microfluidic system is the best compromise, while offering portable and compact systems. Permanent micro-magnets generate their own stray magnetic field and gradient at remanence state, leading to autonomous systems. However, the fabrication of microscale permanent magnets by conventional thin-film techniques requires a precise control over the shape, size and material microstructure and involves complex and expensive processes. In addition, their implementation in polymer-based devices brings up heterogeneous integration issues. To overcome this technological challenge, we present a simple fabrication process based on NdFeB particle self-ordering in a polymer matrix. The NdFeB@polydimethylsiloxane (PDMS) microstructure was simulated on Comsol® to determine the generated magnetic forces. The calculations were consistent with colloidal probe atomic force microscopy (AFM) measurements and showed magnetic forces up to few nN at contact. In particular, we showed that enhanced magnetic forces could be achieved at large distance by adding an external permanent magnet (25x8x2 mm3). Finally, the micro-magnets were integrated at the bottom of a microfluidic channel and efficient magnetic bead trapping was demonstrated thanks to the beneficial effect of two-scale permanent magnets. The integrated permanent micro-magnet array inside a polymer membrane was obtained using a previously described method [2]. The principle rests on an organized agglomeration of magnetic powder inside a polymer matrix by submitting the composite to an external magnetic field pattern during the polymer reticulation. NdFeB particles organize in chain-like agglomerates characterized by a diameter of 5 µm, a length of 75 µm and an areal density of 1500 agglomerates/mm2. The composite membrane was then sealed to a microfluidic channel by O2 plasma bonding. In addition, magnetic force measurements were conducted by colloidal probe AFM to assess magnetic performances of the permanent micro-magnets. Finally, magnetophoretic trapping ability of the micro-magnets, acting as micro-traps, was demonstrated with 12-µm superparamagnetic beads at increasing flow rates (0.5-4 mL/h). In particular, we investigated the effect of adding an external permanent magnet (25x8x2 mm3) at the bottom of the micro-traps on the system magnetic performances. Magnetic field gradients generated by the NdFeB agglomerates were estimated from numerical simulations (Comsol Multiphysics®) and the effect of an external permanent magnet was studied. As illustrated in Figure 1-A, micro-traps alone generate magnetic gradients as high as 105 T/m at contact, 104 times higher than the gradients produced by the external magnet alone, whereas the external magnet permits to reach higher magnetic gradient values for larger distances from the traps (20 µm). These observations were confirmed by colloidal probe AFM measurements. As shown in Figure 1-B, magnetic forces generated by the micro-magnet at contact are comprised between 0.5 and 2 nN, while adding an external magnet permits to widen the force range to 4 nN, which surpasses forces of several pN usually encountered in microfluidic devices. Finally, the micro-traps were integrated in a microfluidic channel and successful capture of superparamagnetic beads was achieved. Trapping efficiency measurements as a function of flow rate are depicted in Figure 1-C. At 1 mL/h, trapping efficiency reached 75% with the micro-traps and the additional external magnet, while it was 50% and 35%, with the micro-traps alone and the magnet alone, respectively. Results demonstrate the benefit of combining micrometer-scale and millimeter-scale permanent magnets to enhance magnetic performances. Indeed, the high magnetic force range of the external magnet attracts beads located in the upper part of the channel while the high localized gradients generated by the micro-traps lead to their trapping. We have presented a low-cost and easy-to-handle fabrication process to integrate large density arrays of permanent micro-magnets in a microfluidic system. In particular, we highlighted the beneficial effect of combining two-scale magnetic sources as strong magnetic field gradients are produced by the micro-magnets and large magnetic force ranges are obtained with an additional external magnet. This novel approach could lead to autonomous and compact magnetophoretic trapping devices.

**References:**

 [1] O. Cugat, J. Delamare, and G. Reyne, IEEE Trans. on Magn. 39, 3607-3612 (2003). [2] L. Descamps, S. Mekkaoui, M.-C. Audry, A.-L. Deman and D. Le Roy, , AIP Adv., 10, 015215 (2020).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/b59a4d294c084f06bca90f3d7475c6d6.jpg)

 High magnetic sorting efficiency in a microfluidic device

Magnetic fluid sealing is a high-performance sealing technique compared to traditional sealing methods, but is limited to a few working conditions. Here we present a novel sealing method using nano-micron magnetic powders as sealing medium. Nano and micron-sized magnetic particles of different materials and size distribution were prepared and mixed. Packing process of magnetic particles was simulated by DEM. Leakage rate was calculated by Darcy’s Law. Finally, the static sealing experiment showed a high pressure resistance and a rather low leakage rate of this novel sealing method. 1. Introduction Sealing with magnetic fluids has been a commonly used high-performance sealing technique with its unique advantages like low friction torque, no leakage and long lifetime. However, it is not capable of resisting high pressure difference or working under extreme temperatures. Magnetic powders are mixture of nano- and micron-sized particles which can be magnetized by an external magnetic field. This research prepared nano-micron magnetic powders of great magnetic and rheological properties. Then the packing process of magnetic powders was simulated and leakage rate was calculated. A static sealing device was designed to test the sealing behavior of this novel sealing method. 2. Methods 2.1 Preparation of magnetic powders Chemical precipitation was used to prepared magnetic nanoparticles. Experimental conditions were adjusted to acquire nanoparticles of different materials, sizes and magnetic intensity. Micron particles were prepared by high-energy ball milling method. Surfactants were coated on particles to control the interparticle forces. 2.2 Simulation of packing process Interparticle forces of magnetic particles are complex and so is the packing process of magnetic powders. In this research, packing process of nano-micron magnetic powders was simulated by DEM using Hertzian-Mindlin model with JKR. Fraction, sizes, magnetization and surfactants of powder preparation, leads to changes of interparticle forces, especially cohesive and magnetic forces. Magnetic particles follow the basic equation: midvi(t)/dt = mig + Fid(t) + Fic(t) + Fim(t) vi(t) is the ith particle's velocity at time t; Fid(t), Fic(t), Fim(t) are the total drag force, net contact force and magnetic force of the ith particle, respectively. 2.3 Calculation of leakage rate by Darcy's Law Darcy's law is commonly used to describe the flow of a fluid through a porous medium. We established a microscopic model to estimate the permeability for calculation of leakage rate of the sealing device. The permeability κ is calculated by the equation: κ = μÛh / △p μ is the fluid viscosity; Û is the outlet velocity; h is the computational zone height; △p is the pressure difference. 2.4 Design and experiments of static sealing device A static sealing device was designed, where magnetic field distribution was simulated to maximize the magnetic flux density in the sealing gap. For each magnetic powder example, sealing capacity and leakage rate were measured. 2.5 Results The simulation results showed that the permeability decreased rapidly with the decrease of particle sizes, and appropriate mixture of nano- and micron-sized particles further decreased the permeability. For the powders prepared, the permeability could be as low as 5.09E-18 m-2. Considering the size of the sealing device, the calculated leakage rate was 2.22E-9 m3/s. Experiments indicated a sealing capacity ≥ 0.5 MPa and leakage rate ≤ 1E-6 Pa×m3/(s×cm). 3. Discussion By selecting magnetic materials, mixture of particles of different sizes and coating particles with surfactants, nano-micron magnetic powders can possess great magnetic and rheological properties. Proper size distribution and interparticle forces lead to a denser packing of magnetic powders. Further, a denser packing leads to a rather low leakage rate, which is verified by both simulation and sealing experiments. A novel sealing method using nano-micron magnetic powders shows distinctive property and may be a promising sealing technique.

**References:**

[1] J. Kafashan, J. Wiacek and N. Abd Rahman, Granular Matter, Vol. 21, p.80 (2019) [2] R.A. Sperling, W.J. Parak, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol. 368, p.1333-1383 (2010) [3] M. Neouze, U. Schubert, Monatshefte für Chemie - Chemical Monthly, Vol. 139, p.183-195 (2008) [4] R. Aboul Hosn, L. Sibille and N. Benahmed, Computers and Geotechnics, Vol. 95, p.30-39(2018) [5] K. Chen, Y. Tian and L. Shan, Smart Materials and Structures, Vol. 22, p.97001 (2013)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/dc98b54b36276cc5bab66afb1c8b1a3a.jpg)
Fig.1 Prepared magnetic powders

![](https://assets.underline.io/uploads/markdown_image/1/image/ab3ec5f9c5d8047ad3aa9245e09e70f4.jpg)
Fig.2 A microscopic model to calculate permeability

 A Novel Sealing Method Using Nano-micron Magnetic Powders and its Leakage Rate Analysis

The transmittable torque of magnetorheological fluid (MRF) clutches can be adjusted by controlling the magnetic field inside the clutch. In order to increase their maximum transmittable torque while decreasing their volume and mass, geometry optimization is often carried out, such as in [1]. Additionally, the MRF used in them can be designed to have a higher yield stress, resulting in a higher transmittable torque. In [2], this was studied by adding micrometer-sized graphite flakes to an in-house created MRF; it was found that the inclusion of graphite flakes led to an increased yield stress. The present research aims to further study this, by investigating the effects that including graphite flakes in a commercially available MRF may have on the transmittable torque of an MRF clutch. Clutch design. The MRF clutch utilized in this research is a drum-type, permanent magnet-based clutch, made from two concentric cylinders. The external cylinder was fabricated from low carbon steel and the internal cylinder was fabricated from aluminum. A cavity for the MRF is formed between these cylinders, with a width of 0.75 mm. The external cylinder formed the main body of the output stage of the MRF clutch. The magnet assembly used to magnetize the clutch consisted of four magnets which have a hollowed-out cylinder shape when assembled together. These magnets were magnetized in the radial direction, with two of them having their south poles in the outer face, while the other two had them in the inner face. The internal cylinder formed the main body of the input stage of the clutch, driven by a DC motor. A mechanism called field blocker, presented in [3], was used to adjust the magnetization level of the MRF inside the clutch. MRF. A commercially available MRF, the LORD MRF-140CG, was used in this study. Four mixtures with 20 μm graphite flakes (282863, Sigma-Aldrich) were prepared, with a graphite content of 0.5% w/w, 1.0% w/w, 2.0% w/w, and 3.0% w/w. Experimental set-up. The MRF clutch was driven by a velocity-controlled DC motor, and its output stage was connected to one end of a 5 Nm T22 HBM torque meter. A lever was attached to the other end of this torque meter, which contacted the testing bench when rotated in one or the other direction. After contacting the bench, this lever forced the output stage of the MRF clutch to remain stationary while the input stage continued rotating. A stepper motor moved the field blocker in or out of the MRF clutch by means of a ball-screw mechanism, covering and uncovering the magnet assembly. The testing set-up is shown in Figure 1 a). Experimental procedure. The testing of the four MRF mixtures together with the MRF without graphite were carried out the following way: 1) The torque output was measured during one second without driving the clutch and with the lever in a free position. 2) The clutch was then driven at the velocity under test for a duration equal to 2 s plus the time required for the clutch to complete a rotation. 3) The clutch was driven backward at a low angular velocity for one second, in order to leave the lever in a free position. This procedure was repeated three times for each combination of five angular velocities, six field blocker positions, with and without the magnet assembly, for a total of 180 measurements per MRF mixture. The average torque value was calculated from the torque recorded during the second testing step, after the first two seconds had passed. The effects on torque due to seal friction and viscosity on the measured torque values were then removed. Results. The results of these tests are summarized in Table 1, Figure 1 b) and c). In Figure 1 b) and c), the average torque values of the tests carried out with the field blocker not covering and covering the magnets are shown (two out of the six positions tested). The torque values are higher in the case were the magnet assembly is uncovered than when it is covered. It is also seen that the difference in torque caused by the graphite content increases as the input angular velocity increases. Higher magnetization levels decrease the difference in torque (the field blocker does not block the magnetic field entirely). The mean, maximum, and minimum torque values over the six magnetization cases are shown in Table 1. It can be seen that on average, the transmittable torque increases with both graphite content and angular velocity, achieving up to 70% torque increase. Conclusion. The inclusion of graphite flakes in a commercially available MRF can be used to increase the output torque of an MRF clutch, a result which aligns to those of [2]. This effect in a clutch with aluminum cylinders is more evident at higher angular velocities.

**References:**

[1] R. M. Andrade, A. B. Filho, C. B. S. Vimieiro, and M. Pinotti, “Optimal design and torque control of an active magnetorheological prosthetic knee,” Smart Mater. Struct., vol. 27, no. 10, p. 105031, 2018. [2] M. K. Thakur and C. Sarkar, “Influence of Graphite Flakes on the Strength of Magnetorheological Fluids at High Temperature and its Rheology,” IEEE Trans. Magn., vol. 56, no. 5, pp. 1–10, 2020. [3] M. A. Fernandez, J. Y. Chang, and C. Y. Huang, “Development of a passive magnetorheological fluid clutch with field-blocking mechanism,” IEEE Trans. Magn., vol. 54, no. 11, pp. 1–5, 2018.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/dd1732b1b4985924706ec3392bdc17b8.jpg)
Table 1. Percentage of the average torque values obtained using MRF with graphite above or below the average torque value obtained with the MRF without graphite.

![](https://assets.underline.io/uploads/markdown_image/1/image/53bb62aa46ec33865da9ddf7f003527d.jpg)
Figure 1. The testing set-up is presented in a). The plotted torque values when the magnet assembly was uncovered and covered are shown in b) and c), respectively.

 A Study on the Effects of Graphite Flakes on Torque Transmission of a Magnetorheological Fluid Clutch

Introduction The manipulation of cell ladened droplets in the bulk culture media is essential for precise analysis and screening of cells, drugs, or other biological objects, which can lead to various applications. With the rapid growth of microfluidic technology, the physical and chemical properties of the droplets can be controlled much more uniformly. This breakthrough leads to more targeted manipulation and handling of the cell-ladened objects. Different methods have been proposed using electrical, optical, or magnetic principles [1]. Among all these methods, magnetic force manipulation is of great interests because it is non-contact, non-thermal, and insensitive to the dielectric properties [2]. The manipulation of magnetic and non-magnetic objects with magnetic fields in various applications have been studied [3]. However, most studies focused on the handling on a single nanoparticle. Little work has been done regarding the tweezing of particle with microscale diameter. Besides, the kinematics mechanism underlying the manipulation of magnetic objects is still not fully studied. This work aimed at developing a platform that can generate and manipulate magnetic-nanoparticle-ladened hydrogel droplets with more controllable manner. The relation between the magnetic force and the behavior of hydrogel droplets was also studied. This work can provide more in-depth understanding of the coupling between magnetic force and fluidic mechanics in handling microfluidic droplets, which can be a guideline in designing magneto-microfluidic platforms for biological applications. Experiments The hydrogel droplets were generated using a microfluidic platform following standard processes [3]. The fabrication of the microfluidic device followed typical soft lithography replica molding technique, by using maskless photolithography and PDMS prepolymer curing [4]. Superparamagnetic nanoparticles (Dynabeads Myone Carboxylic Acid, Purchased from ThermoFisher) was added into the hydrogel solution and mixed completely before the generation of the droplets. The magnetic beads were used as the medium to respond to the external magnetic field; thus the hydrogel droplets can be manipulated remotely. The flow rates of the oil (HFE 7500) and hydrogel solution were carefully adjusted to generate droplets with desired diameter, about 55 um. The generation of the hydrogel droplet is illustrated in Fig. 1a. After the droplets were solidified with fixed shape, the droplets were released into cell culture media, as shown in Fig. 1b. An external magnetic field was exerted using an electromagnet to manipulate the magnetic-nanoparticle-ladened hydrogel droplets within the culture media. The magnetic field could be controlled by adjusting the current of the DC power source. The magnetic manipulation setup and the movement of hydrogel droplets in the culture media are shown in Fig. 1c, Fig. 1d and Fig. 1e. Results and discussion The microfluidic device was successfully fabricated, and the magnetic-nanoparticle-ladened hydrogel droplets were generated using the droplet microfluidic platform. The diameter of the droplet was approximately 55 um, which is suitable for cell encapsulation and other biological applications. After the hydrogel droplets were released into cell culture media, an external magnetic flied was exerted to manipulate the droplets. The results showed that the droplets could be moved in the culture media through magnetic force (Fig. 2a and Fig. 2b). By adopting superparamagnetic nanoparticles, the droplets could be moved rapidly, at a speed of 200 um/s, as shown in Fig. 2c). Since there is no remanence in the superparamagnetic nanoparticles after each manipulation, the hydrogel droplets could be manipulated for several times as desired, which is also favorable for biological applications in which multiple steps are required. In the experiments, the droplets sank to the bottom after 5 min, due to the higher density of the hydrogel droplets relative to water. The moving speed of the hydrogel droplet was much slower than when the droplets were suspended in the media. A detailed kinetics analysis of the movement of hydrogel droplets will be addressed in the full paper.

**References:**

References: [1] Yi, Changqing, et al. "Microfluidics technology for manipulation and analysis of biological cells." Analytica chimica acta 560.1-2 (2006): 1-23. [2] Pamme, Nicole. "Magnetism and microfluidics." Lab on a Chip 6.1 (2006): 24-38. [3] Utech, Stefanie, et al. "Microfluidic generation of monodisperse, structurally homogeneous alginate microgels for cell encapsulation and 3D cell culture." Advanced healthcare materials 4.11 (2015): 1628-1633. [4] Nan, Lang, et al. "Oil-mediated high-throughput generation and sorting of water-in-water droplets." Microsystems & Nanoengineering 6.1 (2020): 1-10. Acknowledgements Yuan Shuai and Prof. Shum provided the idea. Yuan Shuai did the experiments in Microfluidics and Soft Matter Group, the University of Hong Kong. Yuan Shuai composed the paper. Prof. Shum and Dr. Pong directed the project and revised the paper. Information of the authors: Shuai Yuan is with the Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong (ysthoth@connect.hku.hk). Anderson H. C. Shum is with the Department of Mechanical Engineering, The University of Hong Kong, Hong Kong (ashum@hku.hk). Philip W. T. Pong is with the Department of Electrical and Computer Engineering, New Jersey Institute of Technology, and the Department of Electrical and Electronic Engineering, The University of Hong Kong (philip.pong@njit.edu).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/fd782eaeb1cfdd24d1ad098290524f53.jpg)
Fig. 1. a) Generation of magnetic nanoparticle ladened hydrogel droplets using microfluidic platform, b) Release of the hydrogel droplets into the cell culture media. c) Setup of the magnetic manipulation platform. d) Illustration of the movement of hydrogel droplets suspended in the culture media in response to the external magnetic field. e) Movement of hydrogel droplets that sank to the bottom.

![](https://assets.underline.io/uploads/markdown_image/1/image/7ad7f5b67300470f8ae35f503485f209.jpg)
Fig. 2. a) and b) Movement of the magnetic-nanoparticle-ladened hydrogel droplets under magnetic flied. The hydrogel droplets at the bottom of the Petri dish moved much more slowly than the floating droplets. c) Moving speed of the suspended and sank hydrogel droplets, respectively. The measurement of the suspended droplets stopped at 5 s because the droplets moved outside the scope.

 Magnetic manipulation of hydrogel droplets in culture media for biological applications

Magnetofluidic cooling (MFC) is a heat transfer technology to transfer heat using thermomagnetic convection (TMC) of a ferrofluid[1, 2]. The temperature-dependent response of the ferrofluid leads to TMC[2] under the combined effect of thermal and magnetic field gradients. Practical applications require application-specific device design and materials. Solar cells can exhibit significant performance degradation with increasing temperature[3]. Methods reported for cooling, e.g., forced water/air circulation, water spraying, floating tracking concentrating, phase-change materials, and water immersion, show several limitations[4]. Here, we report a novel MFC device for the cooling of solar cells. We first developed a simulation model to investigate the ferrofluid flow needed to cool solar cells. Studying the effect of magnetic field strength, field distribution, channel size, and hydrodynamic properties of the fluid resulted in a novel design (Figure 1). We developed a novel ferrofluid and integrated it with our MFC system. We then built a setup for quantitative measurements of the solar cell performance. As expected, the solar cells showed decreasing performance with increasing temperature without MFC. On the other hand, with MFC (yellow band regions in Figure 2), we observed significant performance improvement. Interestingly, the MFC device demonstrated a higher relative performance enhancement at higher initial solar cell temperatures due to the self-regulating behavior of the MFC device. Hence, the current MFC system offers a novel, versatile, green, energy-efficient, environmentally friendly, noise-free, and vibration-free approach for passive cooling of solar cells. Acknowledgments This research is supported by grants from the National Research Foundation, Prime Minister’s Office, Singapore, under its Campus of Research Excellence and Technological Enterprise (CREATE) programme.

**References:**

[1] V. B. Varma, M. S. Pattanaik, S. K. Cheekati, and R. V. Ramanujan, "Superior cooling performance of a single channel hybrid magnetofluidic cooling device," Energy Convers. Manage., vol. 223, p. 113465, 2020. [2] M. S. Pattanaik, V. B. Varma, S. K. Cheekati, G. Prasanna, N. M. Sudharsan, and R. V. Ramanujan, "A self-regulating multi-torus magneto-fluidic device for kilowatt level cooling," Energy Convers. Manage., vol. 198, p. 111819, 2019. [3] P. Singh and N. M. Ravindra, "Temperature dependence of solar cell performance—an analysis," Sol. Energy Mater. Sol. Cells, vol. 101, pp. 36-45, 2012. [4] J. Siecker, K. Kusakana, and B. Numbi, "A review of solar photovoltaic systems cooling technologies," Renewable Sustainable Energy Rev., vol. 79, pp. 192-203, 2017.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/64997af6a802d48d79de39536f74d9ea.jpg)
Figure 1: (a) Schematic of the application-specific MFC device for Solar cell cooling. (b) Numerical simulations of the MFC device showing temperature profile with and without MFC.

![](https://assets.underline.io/uploads/markdown_image/1/image/4793520fe2d25aa7216475af3a89d499.jpg)
Figure 2: Solar cells surface temperature and generated voltage with and without magnetofluidic cooling (MFC). Yellow bands denote the MFC of the solar cell under an applied magnetic field of 550 mT. Gray bands show the performance without MFC.

 A Novel Magnetofluidic System to Cool Solar Cells

Iron oxide nanoparticles (NPs) have attracted a great attention for biomedical and environmental applications due to their good biocompatibility and magnetic performance. The control over the crystal structure and oxidation state are of paramount importance to optimize their properties [1, 2]. In this work we studied the effect of the amount of both 1,2-hexadecanediol and the solvent 1-octadecene on the thermal decomposition method with iron (III) acetylacetonate. On the one hand, a higher amount of one of the reagents gives rise to single-phase, single-crystal Fe3O4 NPs with diameters below 10 nm and narrow size distributions but with moderate reaction yields. On the other hand, low amounts of either of the two reagents result in large NPs containing both Fe3O4 and FeO phases but with high values of the reaction yield. Consequently, the samples exhibited two distinct magnetic behaviors depending on the amount of these two reagents. The hysteresis loops at room temperature for the small NPs showed the typical features of superparamagnetism: values of the saturation magnetization close to the bulk one for magnetite with no coercive field. On the contrary, larger NPs showed ferrimagnetic behavior with reduced values of the saturation magnetization, as well as shifted hysteresis loops at 5 K after field cooling the sample at 1 T. Zero-field cooling-field cooling (ZFC-FC) curves below 200 K for the small NPs exhibited a peak below room temperature corresponding to the blocking temperature, while those curves for the larger particles displayed two peaks at higher temperatures which can be associated with the Verwey and Neel transitions of magnetite and wüstite phases, respectively. This shows the biphasic nature of the large NPs. With this accurate monitoring of the reaction conditions we have added an extra level of optimization to the synthesis of these NPs. In particular, we have found that, for 1 mmol of iron (III) acetylacetonate, the minimum amounts of 1,2-hexadecanediol and 1-octadecene for the preparation of monophasic, single-crystale Fe3O4 NPs are 2.5 mmol and 5 mL, respectively. This allows us to tune the properties of each sample of iron oxide NPs to its specific application. [3] Acknowledgements The work was supported by Spanish MCIU and AEI (MAT2015-68772-P; PGC2018-097789-B-I00) and European Union FEDER funds. M.E-T. acknowledge Spanish MCIU for BES-2016-077527.

**References:**

[1] A. Fraile Rodríguez, C. Moya, M. Escoda-Torroella, A. Romero, A. Labarta and X. Batlle. J. Mater. Chem. C, 6, 4, 875–882 (2018). [2] C. Moya, M. P. Morales, X. Batlle and A. Labarta. Phys. Chem. Chem. Phys., 17, 19, 13143–13149 (2015). [3] M. Escoda-Torroella, C. Moya, A. Fraile Rodríguez, X. Batlle and A. Labarta. Langmuir (2020) doi.org/10.1021/acs.langmuir.0c02221

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/790431501f371d0ebef8ec3cb20fc3ab.jpg)
Colored HRTEM images of two samples synthetized with increasing amounts of 1,2-hexadecanediol and 1-octadecence and their respective magnetization curves, showing the correlation between low amount of both reagents and a defective crystal structure and a poorer magnetic behavior.

 Selective Control of Reagents as a Tuning Knob for Iron Oxide Nanoparticles with Controlled Morphology, Oxidation State and Magnetic Response

Cobalt ferrite nanoparticles (NPs) are competitive candidates in nanomedicine and biomedical applications, especially in the fields of magnetic drug delivery, magnetic resonance imaging, and separation and detection of biomolecules [1,2]. The reason behind is a suitable combination of excellent chemical and magnetic characteristics, such as a high chemical stability, surface active sites, and ease of synthesis and functionalization, together with a high anisotropy constant, a high coercivity, and a moderate saturation magnetization. However, in order to control the functional response of cobalt ferrite NPs, one of the biggest challenges is to quantitatively disentangle the dependence of the composition, structure, or surface chemistry onto the overall magnetic response. In this study we unravel the key role played by the Co2+ cations on the destabilization of the collinear ferrimagnetism in cobalt ferrite NPs, by combining an advanced synthesis approach with a broad set of world-class complementary local probes. A set of samples of monodispersed, 8 nm cobalt ferrite NPs of identical stoichiometry but with a progressive inclusion of structural defects was prepared [3,4]. SQUID magnetometry results show a rapid degradation of the collinear ferrimagnetism as the structural disorder increases within the NPs, and even samples that are almost free from crystallographic defects exhibit relatively large values of the high-field susceptibility suggesting the occurrence of canting at least for some of the cations and sites [5]. As local characterization probes, synchrotron-based, element-, valence- and site- specific X-ray spectroscopy and magnetometry on ensembles of NPs was combined with high resolution transmission electron microscopy of selected, individual NP (Fig. 1) [5]. The analysis of element-specific X-Ray Magnetic Circular Dichroism (XMCD) spectra and hysteresis loops for all cationic sites reveals that the collinear alignment of the Co2+ cations in octahedral sites is significantly more affected by the structural disorder than in any other cation. This is because structural defects cause local distortions of the crystal field acting on the orbital component of the cations, yielding effective local anisotropy axes that cause a prevalent Co2+ spin canting through the spin–orbit coupling, owing to the relatively large value of the partially unquenched moment of these cations, as found by XMCD. As the number of structural defects increases, the rest of the cations are progressively dragged off the ferrimagnetic alignment, being the Fe3+ cations in tetrahedral sites the last ones to be affected by the disorder because the canting takes place first in octahedral sites thanks to their smaller number of next-nearest neighbors in the tetrahedral sublattice [5]. Our results demonstrate the key role of the Co2+ cations on the destabilization of the collinear ferrimagnetism in Co-ferrite NPs as their crystalline quality worsens and may help clarify the often conflicting, large variability of magnetic properties in the literature of Co-ferrite NPs with slightly different structural features. Our work may provide new avenues to interpret and control the functional response of Co-ferrite NPs, of relevance in the fields of rare-earth free permanent magnets, biological separation and detection, or nanomedicine. Acknowledgements The work was supported by Spanish MCIU and AEI (MAT2015-68772-P; PGC2018-097789-B-I00) and European Union FEDER funds. M.E.T. acknowledges Spanish MINECO for the Ph.D. contract BES-2016-077527. A.F.R. and C.M. acknowledge the financial support from the EU CALIPSO Transnational Access programme. Part of this work was performed at the Swiss Light Source, Paul Scherrer Institut, Switzerland. The measurements at FEI TITAN3 of the Laboratorio de Microscopias Avanzadas (LMA), Instituto de Nanociencia de Aragon (INA), Universidad de Zaragoza, are also gratefully acknowledged. S.R.V.A. thanks the Swiss National Science Foundation for financial support under project Nr. 169467.

**References:**

[1] S. Y. Srinivasan, K.M. Paknikar, D. Bodas, et al. Applications of Cobalt Ferrite Nanoparticles in Biomedical Nanotechnology. Nanomedicine. Future Medicine Ltd. May 2018, pp 1221–1238. https://doi.org/10.2217/nnm-2017-0379. [2] S. Amiri, H. Shokrollahi, The Role of Cobalt Ferrite Magnetic Nanoparticles in Medical Science. Materials Science and Engineering C. Elsevier January 2013, pp 1–8. https://doi.org/10.1016/j.msec.2012.09.003. [3] C. Moya, M. D. P.Morales, X. Batlle, A. Labarta, Phys. Chem. Chem. Phys. 17, 13143 (2015). [4] C. Moya, G. Salas, M. D. P. Morales, et al. J. Mater. Chem. C 3, 4522 (2015). [5] C. Moya, A. Fraile Rodríguez, M. Escoda-Torroella, et al., J. Phys Chem. C (2020) (in press). https://dx.doi.org/10.1021/acs.jpcc.0c06657.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/176a07ed0dc34fb44fc96344b80c0136.jpg)
Fig. 1. Monodisperse samples of 8 nm NPs with a tunable amount of structural defects prepared by thermal decomposition of Fe(III) and Co(II) acetylacetonates with a variable concentration of 1,2-hexadecanediol. HRTEM images and element-specific XMCD loops of the Co2+ cations corresponding to highly defective (a, c) and highly crystalline nanoparticles (b, d).

 Crucial Role of the Co Cations on the Destabilization of the Ferrimagnetic Alignment in Co-Ferrite Nanoparticles with Tunable Structural Defects

The heating ability of magnetic nanoparticles when subjected to alternating magnetic fields is a topic of interest for biomedical applications, as in the case of magnetic hyperthermia, and more recently in the area of induction heating in heterogeneous catalysis [1-2]. In particular, AC magnetometry stands out as a powerful technique to study the dynamic hysteresis loops of the nanoparticles in the desired medium and to quantify their heating efficiency, which is proportional to the area of the cycles. When properly calibrated, this device provides more accurate and reproducible measurements than the most widespread calorimetric method, giving also information about the magnetic properties of the system such as the coercive field, magnetization saturation and susceptibility. The study of the dynamic behaviour of systems of magnetic nanoparticles under radiofrequency fields is still being investigated both experimentally and theoretically [3-5], in particular the influence of nanoparticle dipolar interactions and chain formation, due to the huge increase in the heating efficiency reported in the literature. In this work, we perform an experimental systematic study of three samples of magnetite with sizes of 12 nm, 30 nm and 50 nm as a function of the applied field, the initial temperature of the sample and of the acquisition time. Time-dependent high-frequency hysteresis loop measurements are presented, showing that, in some cases, the hysteresis strongly depends on the acquisition time of the cycle, effect that can be associated to the chain formation. In the case of the 30 nm sample, there is a strong dependence of the hysteresis cycle shape with the acquisition time, increasing the area as well as the susceptibility as the acquisition time increases; whereas the 50 nm nanoparticles show always the same hysteresis cycles independently of the acquisition time (see Fig. 1). This effect is associated to the time required for the chain formation. Similar results have been already observed by Asensio et al., in iron carbide nanoparticles [1].

**References:**

[1] J.M. Asensio, et al., Nanoscale., Vol. 11(12), p.5402-5411 (2019) [2] B. Rivas-Murias, et al., Angewandte Chemie., Vol. 132(36), p.15667-15672 (2020) [3] I. Morales, et al., Nanomaterials., Vol. 8(12), p.970 (2018) [4] I. Rodrigo, I. Castellanos-Rubio, E. Garaio, et al., International Journal of Hyperthermia., Vol. 31(1), p.976-991 (2020) [5] D.P. Valdés, E. Lima Jr, R.D. Zysler, E. De Biasi, Physical Review Applied., Vol. 14, p.014-023 (2020)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/2f6401c65514349ed6ceb4b1826c577f.jpg)
Fig. 1. Evolution of the cycles as a function of the acquisition time between 5 ms and 10 s, for an applied field of 27 mT and 50.3 kHz.

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