Jason S. Wexler

Conformal coating of particles in microchannels by magnetic forcing

We present a co-flow microfluidic method to coat paramagnetic beads with a thin layer of fluid as the beads are pulled across a liquid-liquid interface by an external magnetic field. We show that the coating thickness can be controlled by the magnitude of the flow speed. Also, the number of beads aggregated within a single coating can be adjusted by varying the strength of the magnetic field or the liquid-liquid interfacial tension.

Experimental study on penny-shaped fluid-driven cracks in an elastic matrix

When a pressurized fluid is injected into an elastic matrix, the fluid generates a fracture that grows along a plane and forms a fluid-filled disc-like shape. We report a laboratory study of such a fluid-driven crack in a gelatin matrix, study the crack shape as a function of time and investigate the influence of different experimental parameters such as the injection flow rate, Young’s modulus of the matrix and fluid viscosity. We choose parameters so that effects of material toughness are small. We find that the crack radius R(t) increases with time t according to tα with α=0.48±0.04. The rescaled experimental data at long times for different parameters collapse based on scaling arguments, available in the literature, showing R(t)∝t4/9 from a balance of viscous stresses from flow along the crack and elastic stresses in the surrounding matrix. Also, we measure the time evolution of the crack shape, which has not been studied before. The rescaled crack shapes collapse at longer times and show good agreement with the scaling arguments. The gelatin system provides a useful laboratory model for further studies of fluid-driven cracks, which has important applications such as hydraulic fracturing.

Overflow cascades in liquid-infused substrates

Liquid-infused patterned surfaces offer a promising new platform for generating omniphobic surface coatings. However, the liquid infused in these surfaces is susceptible to shear-driven dewetting. Recent work [Wexler et al., “Shear-driven failure of liquid-infused surfaces,” Phys. Rev. Lett. 114, 168301 (2015)] has shown how the substrate pattern in these surfaces can be designed to exploit capillary forces in order to retain infused lubricants against the action of an immiscible shear flow. In this study, we explore the behavior of the infused lubricant when external shear causes the lubricant to overflow finite or “dead-end” surface features, resulting in either temporary or permanent lubricant loss. Microfluidic experiments illustrate how both geometry and chemical Marangoni stresses within liquid-infused surfaces generate an overflow cascade in which the lubricant escapes from the substrate and forms droplets on the surface, after which the droplets depin and are washed away by the external shear flow...

Crossover from shear-driven to thermally activated drainage of liquid-infused microscale capillaries

The shear-driven drainage of capillary grooves filled with viscous liquid is a dynamic wetting phenomenon relevant to numerous industrial processes and novel lubricant-infused surfaces. Prior work has reported that a finite length

Stratified thin-film flow in a rheometer

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Separation of Magnetic Beads in a Microfluidic Device: Modeling and Experimentation

of the capillary groove can remain indefinitely filled with liquid even when large shear stresses are applied. The mechanism preventing full drainage is attributed to a balance between the shear-driven flow and a counterflow driven by capillary pressures caused by deformation of the free surface. The final equilibrium length

Shear-driven failure of liquid-infused surfaces.

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Robust liquid-infused surfaces through patterned wettability

is uniquely determined by physical properties of the filling liquid as well as the geometry and wettability of the capillary. In this work, we examine closely the approach to the final equilibrium length

Microfluidic ultralow interfacial tensiometry with magnetic particles.

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Bending of elastic fibres in viscous flows: the influence of confinement ‡

and report a crossover to a slow drainage regime that cannot be described by conventional dynamic models considering solely hydrodynamic and capillary forces. The slow drainage regime observed in experiments can be instead modeled by a kinetic equation describing a sequence of random thermally activated transitions between multiple metastable states caused by surface defects with nanoscale dimensions. Our findings provide new insights on the critical role that natural or engineered surface roughness with nanoscale dimensions can play in the imbibition and drainage of capillaries and other dynamic wetting processes in microscale systems.