Session H18: Microfluids: General V

Hydrodynamic Coupling of Small Clusters of Particles in a Narrow Channel

Control of flowing suspensions is central to many emerging microfluidic applications. For instance, manipulation of small clusters is important in the synthesis of functional particles. Via theory, simulations, and experiment, we study small clusters confined in a microchannel with thin cross section and subject to an external flow. Our stop flow lithography (SFL) technique uniquely allows for precise control over particle shape, rigidity, and initial placement. We show that many-body hydrodynamic interactions sustain long-lived bound states with complex dynamics. As these interactions are sensitive to confinement, we investigate modulation of channel geometry as a means to perform sequential operations in a continuous process. We also probe the effects of shape and rigidity via SFL and a Lattice Boltzmann/Lattice Spring code, finding that near-field and orientational effects of shape enrich behavior. For soft, extended particles, we probe hydrodynamic self-deformation and self-excitation of elastic modes. Our results demonstrate phenomena that could be exploited for assembly of soft colloids in microchannels.   

Analysis of pinching in deterministic particle separation

We investigate the problem of spherical particles vertically settling parallel to Y-axis (under gravity), through a pinching gap created by an obstacle (spherical or cylindrical, center at the origin) and a wall (normal to X axis), to uncover the physics governing microfluidic separation techniques such as deterministic lateral displacement and pinched flow fractionation: (1) theoretically, by linearly superimposing the resistances offered by the wall and the obstacle separately, (2) computationally, using the lattice Boltzmann method for particulate systems and (3) experimentally, by conducting macroscopic experiments. Both, theory and simulations, show that for a given initial separation between the particle centre and the Y-axis, presence of a wall pushes the particles closer to the obstacle, than its absence. Experimentally, this is expected to result in an early onset of the short-range repulsive forces caused by solid-solid contact. We indeed observe such an early onset, which we quantify by measuring the asymmetry in the trajectories of the spherical particles around the obstacle.   

Manipulation of suspended microparticles by steady streaming

It is well known that a body oscillating in a viscous fluid will generate steady streaming cells, a weak secondary flow created by non-linear interactions of the primary oscillatory flow. In this work, we explore the manner in which this steady streaming can transport microscale inertial particles. We use high-fidelity numerical simulations solving Navier-Stokes equation to simulate the steady streaming generated by a cylindrical probe undergoing translational oscillations perpendicular to its axis in a viscous fluid. It is observed that inertial particles are attracted toward small-amplitude limit cycles in one of the four after traveling a spiral trajectory. We especially focus on the influence of physical parameters such as inertial particle's size and density and probe oscillation frequency and amplitude on the streaming structures and attracting speed of the inertial particle. We also study the behaviors of particle motion in the vicinity of multiple probes, particularly when the oscillation parameters of the probes are asymmetric.   

Oscillations of a fiber flowing in a confined microchannel

Transport of slender bodies in confined geometries is of interest in various industrial applications. In the oil industry, fibers are widely used for stimulation or to prevent losses into the rock formations. Applications can also be found in biology systems such as targeted drug delivery. We present an experimental and numerical investigation of the flow of an advected fiber in a confined microchannel. The fiber is fabricated \textit{in situ} using a photo-polymerization method to ensure an excellent control of its geometry and its mechanical properties. When imposing a constant flowrate, we observe that the fiber oscillates continuously between the lateral walls until it exits the channel. We characterize the oscillation period as a function of the flow velocity, the fiber length and channel width. This phenomenon can be used to generate efficient mixing at the microscale.   

Determining the equilibrium distribution of particles in nanofluidic systems

We study the equilibrium distribution of particles between plates with separation of less than one micron, which is relevant to many colloidal and biomolecular systems. We compare two common simplifying approximations, the linear superposition model and the ion model, to the full nonlinear Poisson-Boltzmann equation. We identify regions in which these simplified models apply, and we describe when and why they fail. Finally, we show that fitting a single parameter in a simplified model will cause it to agree with the full model, implying that even a believable fit is not evidence for a model's validity.   

Theory of viscous corrections to the acoustic radiation force on a suspended microparticle in a standing ultrasound wave

We present a theoretical analysis of the acoustic radiation force causing acoustophoresis on suspended microparticles and cells in an standing ultrasound field of frequency $\omega$. We include the kinematic viscosity $\nu$ of the solvent thereby extending the now classical and widely used theory by Gorkov valid only for inviscid solvents [1]. The viscosity appears through the formation of the incompressible viscous boundary layer of width a few times $\delta = \sqrt{2\nu/\omega}$ around the suspended particle. Previous analyses [2,3] of the dependence of $\delta$ had emphasis on developing general theoretical schemes and provided analytical expressions only in the limit $\delta \ll a \ll \lambda$. Our analysis does not have this limitation, and we take into account the incompressible boundary layer surrounding the particle and where viscosity dominates, and match the acoustic wave here with that in the compressible solvent where viscosity can be neglected. We apply our analytical result to calculate the values of the viscous corrections for particles of size and composition typically employed in microchannel acoustophoresis.\\[2mm] {}[1] L.P. Gorkov, Sov.~Phys.~Doklady \textbf{6}, 773 (1962).\\ {}[2] A.A. Doinikov. J.~Acoust.~Soc.~Am.~\textbf{101}(2), 722 (1997).\\ {}[3] S.D. Danilov, M.A. Mironov. J.~Acoust.~Soc.~Am.~\textbf {107}(1), 722 (2000).   

Particle-size dependent cross-over from radiation-dominated to streaming-dominated acoustophoresis in microchannels

Expanding the use of microchannel acoustophoresis to handle particles smaller than 1~$\mu$m is a challenge due to the particle-size dependent cross-over from the well-understood radiation-dominated motion of microparticles to the ill-characterized streaming-dominated motion of submicron particles. Using our newly-developed, automated, temperature-controlled, and high-precision micro-PIV system (Augusstson \textit{et al}., Lab Chip, submitted 2011), we measure the acoustophoretic velocity fields of polystyrene particles in the range from 10~$\mu$m to below 1~$\mu$m. We use the Helmholtz decomposition theorem and discrete Fourier transform to decompose each velocity field into a gradient part from radiation and a rotation part from streaming. From the decomposed velocity fields, we obtain as expected that the particle velocity induced by acoustic radiation scales with the particle size to the power 2, while the acoustic-streaming-induced particle velocity is independent of the particle size (Barnkob \textit{et al}., Proc.\ 14th MicroTAS 2010, p.~1247). Furthermore, we study the theoretical prediction that the critical particle size for cross-over scales linearly with the width of the viscous boundary layer and thus scales linearly with buffer viscosity and inversely with actuation frequency.   

Microfluidic modeling of the effects of nanoparticles on the blood-brain barrier in flow

The difficulty of diffusing drugs across the blood-brain barrier (BBB) has caused an impasse for many brain treatments. Nanoparticles (NPs), to which drugs can adsorb, attach, or be entrapped, have the potential to deliver drugs past the BBB. Before nanoparticles can be used, their effects on the BBB and brain must be ascertained. Previous steady-state studies fall short for closely modeling \textit{in vivo} conditions$.$ Convection of nanoparticles is ignored, and endothelial cells' (ECs) morphology differs based on loading conditions; \textit{in vitro} loading with continuous flow exhibit ECs indicating a more similar \textit{in vivo} phenotype. NPs interact with monocytes prior to the BBB, and their toxicity effects were measured in flow conditions using both Trypan Blue cell counting and cell proliferation assays. The microfluidic device designed to model the BBB contained a concentric PES hollow fiber porous membrane in PFA tubing. Full use of the device will include ECs adhered on the inner surface and astrocytes adhered to the outer surface of the PES membrane to model cerebrovascular capillaries.   

Diamagnetic Particle Deflection in Ferrofluid Flows through a Rectangular Microchannel

Magnetic field-induced particle manipulation is a promising technique for biomicrofluidics applications and offers several advantages over other traditional approaches based on electric, acoustic and optical forces. We present in this talk a fundamental study of diamagnetic particle motion in ferrofluid flows through a rectangular microchannel with a nearby permanent magnet. Due to their negligible magnetization relative to the ferrofluid, diamagnetic particles experience negative magnetophoresis and are repelled away from the magnet. The result is a three-dimensionally focused particle stream flowing near the bottom outer corner of the microchannel. The effects of particle size and position, ferrofluid flow rate and concentration, and magnet-channel distance on the diamagnetic particle deflection are systematically studied. The obtained experimental results agree reasonably with the predictions of a developed three-dimensional analytical model.