Session F16: Microscale Flows: Oscillations

Density-contrast induced inertial forces on particles in oscillatory flows: quantifying an efficient tool in microfluidics and acoustofluidics.

Oscillatory flows have become an indispensable tool in inertial microfluidics, exerting significant and consistent forces on fluid-borne objects. The use of localized oscillating objects to precisely manipulate particles is now pushing the envelope of existing theories, which are unable to account for experimental observations. While earlier work formalized an unrecognized attractive contribution towards oscillating interfaces acting even on neutrally buoyant particles, here we quantify additional inertial forces on particles due to a finite density contrast. Through a rigorous analytical modeling approach we find that these forces emerge from an interplay between particle inertia, slip velocity and flow gradients, and are important, in particular, for nearly density matched cell-sized particles in biomicrofluidics, where they can be used for fast displacement and separation. We further show that the Auton modification to the added mass term in the Maxey-Riley equation naturally emerges from our theory as a limiting case. Our formalism also generalizes the far-field acoustofluidic secondary radiation force on particles in inviscid flows to include viscous effects, thus bridging the two fields. These predictions are confirmed against independent direct numerical simulations.   

Modulating rectified flows via elastohydrodynamic effects

Rectified flows associated with the oscillatory motion of immersed rigid boundaries, known as viscous streaming flows, represent an efficient way of manipulating and controlling fluids via inertial effects. Despite their potential, we know surprisingly little about streaming when body compliance is involved, a situation frequently encountered in microfluidic and biological settings. Motivated by this, we conduct the first theoretical investigation into body compliance-induced streaming, via the minimal case of an oscillating, hyperelastic cylinder immersed in a viscous fluid. Further, we confirm our findings against direct numerical simulations. Our study demonstrates the existence of a new elasticity-induced term, allowing forms of inertial flow control, even in the Stokes limit, unavailable in the case of rigid bodies.   

Characterizing the Rich Variety of Streaming Flow

Steady streaming flow is the result of nonlinear interaction of periodic flow components, used in wide-ranging applications to manipulate positions of particles in microfluidic set-ups. Originating from the oscillatory motion of an interface, the periodic flows can be systematically decomposed e.g. into multipolar components. Self-interaction of modes (inducing single-mode streaming) as well as interaction between distinct modes (inducing mixed-mode streaming) results in qualitatively different spatial patterns of streaming flows, some of which show robust vortex patterns, while others display intricate radial and angular dependencies that can be strongly shifted by even small changes of control parameters such as the relative phases of mixed modes. We demonstrate this rich variety of streaming experimentally using both oscillatory motion over a solid object and oscillatory motion of a fluid-fluid interface. A systematic approach to predicting diverse manifestations of streaming depending on the oscillation modes and boundary conditions is developed, using asymptotic matching in the regime of small streaming Reynolds numbers. The formalism gives indications as to which modes (volumetric, translational, shape) should be combined to obtain a desired streaming pattern for optimal use in particle concentration, separation, or aggregation, and thus provides practical guidelines for device design.   

Orchestrating rectified flows via dynamic morphological variation

A potential explanation for observed flows actively generated by ciliated microorganisms lies in inertial rectification, through a mechanism known as viscous streaming. While conventional streaming theory is well understood for static shapes, it fails to capture effects arising from dynamic morphological variations characteristic of biological settings. Motivated by this, we theoretically investigate these phenomena via the classical 2D squirmer model in Stokes-like regimes, and confirm our findings against direct numerical simulations. Our study elucidates a previously unreported shape-mode effect responsible for flow rectification, providing a potential rationale for the robust and controllable flows exhibited by a variety of aquatic microorganisms.   

Smoothing oscillatory peristaltic pump flow with non-linear passive components

We have explored the effect of adding linear, non-linear resistors and capacitors in novel combinations to selectively smooth the flow from a peristaltic pump. With this novel approach we have reduced our oscillation amplitudes to 0.05mL/min for a 6mL/min flow rate. We have achieved this with the construction of 3D printed millifluidic non-linear valves and produced an effective predictive model for the behaviour of these devices. This valve has then been coupled to the flow from a peristaltic pump with capacitors and a linear resistor. We have compared these results to a predictive model of peristaltic pump flow which qualitatively predicts the behaviour of various configurations of the components and allows further development of intelligently designed flow networks incorporating pulsating flow.   

Microgravitational Particle Interaction in Monochromatic High-Frequency Oscillatory Flow

Particle-resolved Direct Numerical Simulations (pr-DNS) are performed to investigate the interaction of two spherical particles submerged in a viscous fluid and subjected to a monochromatic high-frequency oscillatory flow in the absence of gravity. This fundamental research serves the main objective of studying the flocculation behavior of clay suspensions in a microgravity environment. The numerical setup is based on experiments previously conducted by the same authors onboard the International Space Station (ISS). The applied range of frequencies corresponds to onboard vibrations (g-jitter) caused by equipment operations and routine crew activity. The simulations account for the fluid-particle interaction via the Immersed Boundary Method (IBM) by resolving the flow field around the suspended particles. In this talk, we will present a detailed sensitivity analysis of the relevant parameters leading to attractive or repulsive interactions between two particles. First results show that the data collapses using an appropriate non-dimensional number reflecting the relevant physical effects of inertia, viscous, and pressure forces.