Session S36: Microscale Flows: Opto-Fluidics, Electro/Magnetic Manipulation

Dynamics of a pair of elliptical microparticles under a uniform magnetic field

Under a uniform magnetic field, non-spherical magnetic particles dispersed in a viscous liquid medium show various rheological properties. However, the fundamental mechanism of the particle-particle interactions of non-spherical particles under the uniform magnetic field is still missing. In this work, we creates two-dimensional direct numerical simulations to investigate the particle-particle interactions and relative motions of a pair of paramagnetic elliptical particles. The direct numerical simulations are based on the finite element method, which used an arbitrary Lagrangian-Eulerian approach with a full consideration of particle-fluid-magnetic field interaction. We investigated the effects of initial position and aspect ratio of the particles on particle-particle interactions and relative motions. The results show that the particles spend much more time for the global reorientation than for the local magneto-orientation. It requires more time to form a stable chain for larger initial relative angles and distances, and larger aspect ratios. The results in this work provide insights on the particle alignment and chaining processes under uniform magnetic fields, which are closely related to the performance and behavior of magnetorheological fluids.   

Leveraging viscous peeling in soft actuators and reconfigurable microchannel networks

The research fields of microfluidics and soft robotics both involve complex small-scale internal channel networks, embedded within a solid structure. This work examines leveraging viscous peeling as a mechanism to create soft actuators and microchannel networks, including complex elements such as valves, without the need for fabrication of structures with micron-scale internal cavities. We consider configurations composed of an internal slender structure embedded within another elastic solid. Pressurized viscous fluid is introduced into the interface between the two solids, thus peeling the two elastic structures and creating internal cavities. Since the gap between the solids is determined by the externally applied pressure, the characteristic size of the fluidic network may vary in time and be much smaller than the resolution of the fabrication method. This work presents a model for the highly nonlinear elastic-viscous dynamics governing the flow and deformation of such configurations. Experimental demonstrations of micron-scale valves and channel-networks created from millimeter scale structures are presented, as well as the transient dynamics of viscous peeling based soft actuators. The experimental data is compared with the suggested model, showing very good agreement.   

Three-dimensional rotation of paramagnetic prolate spheroids in simple shear and uniform magnetic field

We present a theoretical investigation on a time-dependent, three-dimensional rotation of an ellipsoidal paramagnetic particle. The particle is placed in an unbounded Newtonian shear flow at zero-Reynolds number and under a uniform magnetic field. In the absence of a magnetic field, the particle will perform periodic rotations known as Jeffery's Orbit. When a magnetic field is applied, we determine the critical field strength that pins the particle's rotation and describe its rotational dynamics towards a stable steady angle when exposed to a strong field (i.e., above the critical field strength). In a weak magnetic regime (i.e., below the critical field strength), we discuss how the paramagnetic particle's polar angle oscillates toward the magnetic field plane, and the time that these oscillations occur has been analyzed. We determine the amplitude of the oscillations and approximate the particle's quasi-steady state of its polar angle. We discuss the relation between the particle's polar angle and the symmetrical rotation of its azimuthal angle, which is affected by the direction of the uniform magnetic field.   

Nonequilibrium all-aqueous fiber formation

We study in-flow equilibration process of aqueous two-phase system (ATPS) of poly(ethylene glycol) diacrylate (PEGDA) and dextran (DEX), and we utilize these nonequilibrium systems to generate structured microfibers. A jet of a PEGDA-rich phase is formed in a continuous DEX-rich phase, inside a microfluidic device. As the ATPS equilibrates, a variety of microstructures evolve. These evolving fluid structures can be used as liquid templates to tune the size or the internal and surface microstructures of PEGDA fibers upon photopolymerization of the jet. We study the dynamics of the equilibration process, and the transient evolution of the liquid microstructures, by tuning parameters such as the initial composition of the phases and flow rates. We anticipate that this nonequilibrium all-aqueous fiber formation system may have important biotechnological applications, specifically for generation of scaffolds of different microstructures. The all-aqueous nature of this system allows for generation of biocompatible materials, not requiring washing steps needed for fibers based on water-oil systems. Moreover, the ultra-low interfacial tension of the ATPS, which is three orders of magnitude lower than that of water-oil, allows for facile generation of microfibers.   

Selective Control of Particle Flows Through Microchannel Contractions by Using Laser-Induced Thermophoresis

Absorption of a focused laser into solvents can be used to create a microscale heat source in microchannels. A temperature gradient of about 1 K/$\mu $m can be achieved due to the smallness of heat source volume, and it effectively exerts thermophoretic forces on dispersed particles to transport them along the temperature gradient The magnitude and direction of the thermophoretic force are sensitive to the physical properties of particle. Therefore, when the mixture of particles with different thermophoretic characteristics are dispersed in the solution, selective particle flows can be realized by using laser-induced thermophoresis. In this study, the demonstration of selective particle transport in a microchannel contraction will be made for the mixture of polystyrene and silica beads with equal diameters. The polystyrene beads are thermophoretically-active and repelled from high temperature regions, while the silica beads are thermophoretically-inactive. By designing the microchannel contraction so that the width is comparable to the laser spot diameter and by irradiating the laser at the contraction entrance, the thermophoretically-inactive silica beads can be transported into the contraction part using the pressure-driven flows while the thermophoretically-active polystyrene beads are moved away from the focal point in two-dimensional axial symmetry.   

Microfluidic experiment and simulation technique for the measurement and representation of particle deformation during three-dimensional bioprinting

Recent advances in biofabrication allow for the production of organs constructs in vitro, which can mimic the microenvironments of native tissues. One of the most promising biofabrication methods is the extrusion-based bioprinting, which utilizes additive-manufacturing concepts to deposit bioinks laden with living cells. However, the survivability of the cells was found to be reduced by the extrusion process. To enhance their survival rate, the cells can be enclosed in protective particles. Still, the particles’ protection may be rendered ineffective by high flow shear-stress, which is challenging to measure. Therefore, a microfluidic experiment that can represent the bioprinting setup was developed. As the bioink was pushed through the microfluidic chip at flow-rates equivalent to the bioprinting, the particle deformations were captured using a microscope, from which stresses can be derived. To this end, several experiments at different conditions have been completed, demonstrating the feasibility of the measurement procedures. Image-processing techniques are being developed to systematically analyze the particle deformations. In addition, smoothed particles hydrodynamics is used to simulate the experiment to explore its applicability in describing the problem.   

Superparamagnetic colloids under rotating magnetic field

Colloids made of superparamagnetic micrograins offer the possibility to be controlled remotely with a magnetic field. This remote control enables various applications such as mixing, transport of fluids through microscale channels or propulsion in a viscous fluid. It is well known that when a constant magnetic field is applied on superparamagnetic colloids, they form long chains aligned with the field. When the magnetic field is rotating, a competition between magnetic and viscous forces takes place, leading to the rotation of the chains following the magnetic field along with the reduction of their length. The frequency of the rotating magnetic field is a key factor in these dynamics. When it is increased, we observe the formation of rotating clusters with frequencies that differ from the one of the field applied. In our work, we studied the influence of the field rotational frequency on the length and rotational speed of the chains as well as on the main features of the clusters. In order to have the needed temporal resolution, we used a microscope combined with a high-speed camera. Thanks to this technology, we were able to determine a critical frequency between the two regimes mentioned above.   

Lateral migration of an electrophoretic particle in Newtonian and viscoelastic pressure driven flows

In recent years there has been a growing interest towards incorporation of electrokinetics to inertial and viscoelastic focusing techniques to achieve higher throughput and external control over the focusing positions. This is performed by applying a DC electric field parallel to the flow. In this work, we investigate the effects of electrokinetics on the particle migration in pressure-driven flows of Newtonian and viscoelastic fluids through experiments and theoretical analysis. Using Lorentz reciprocal theorem in conjunction with perturbation expansion (in Reynolds or Deborah number), we derive analytical expressions which describe the lateral migration at the leading order. We find that that the direction of migration in inertial-Newtonian flow and inertialess-viscoelastic flow is towards the regions of high shear, provided the electrokinetic motion is in the direction of the imposed Poiseuille flow. The trajectories obtained from the theoretical analysis, in agreement with experiments, demonstrate that the interaction of electrokinetic and rheological effects can result in an enhancement in migration by an order of magnitude. Furthermore, it is revealed that the magnitude of the background shear and particle zeta potential primarily governs the migration.