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1. Surface corrugations induce helical near-surface flows and transport in microfluidic channels

   https://doi.org/10.1017/jfm.2024.106  

   Kurzthaler, Christina; Chase, Danielle L; Stone, Howard A (March 2024, Journal of Fluid Mechanics)

   We study theoretically and experimentally pressure-driven flow between a flat wall and a parallel corrugated wall, a design used widely in microfluidics for low-Reynolds-number mixing and particle separation. In contrast to previous work, which focuses on recirculating helicoidal flows along the microfluidic channel that result from its confining lateral walls, we study the three-dimensional pressure and flow fields and trajectories of tracer particles at the scale of each corrugation. Employing a perturbation approach for small surface roughness, we find that anisotropic pressure gradients generated by the surface corrugations, which are tilted with respect to the applied pressure gradient, drive transverse flows. We measure experimentally the flow fields using particle image velocimetry and quantify the effect of the ratio of the surface wavelength to the channel height on the transverse flows. Further, we track tracer particles moving near the surface structures and observe three-dimensional skewed helical trajectories. Projecting the helical motion to two dimensions reveals oscillatory near-surface motion with an overall drift along the surface corrugations, reminiscent of earlier experimental observations and independent of the secondary helical flows that are induced by confining lateral walls. Finally, we quantify the hydrodynamically induced drift transverse to the mean flow direction as a function of distance to the surface and the wavelength of the surface corrugations. 

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2. Simulated surface diffusion in nanoporous gold and its dependence on surface curvature

   https://doi.org/10.1016/j.commatsci.2023.112430  

   Winkeljohn, Conner Marie; Shahriar, Sadi; Seker, Erkin; Mason, Jeremy K (October 2023, Computational Materials Science)

   The morphological evolution of nanoporous gold is generally believed to be governed by surface diffusion. This work specifically explores the dependence of mass transport by surface diffusion on the curvature of a gold surface. The surface diffusivity is estimated by molecular dynamics simulations for a variety of surfaces of constant mean curvature, eliminating any chemical potential gradients and allowing the possible dependence of the surface diffusivity on mean curvature to be isolated. The apparent surface diffusivity is found to have an activation energy of ~0.74 eV with a weak dependence on curvature, but is consistent with the values reported in the literature. The apparent concentration of mobile surface atoms is found to be highly variable, having an Arrhenius dependence on temperature with an activation energy that also has a weak curvature dependence. These activation energies depend on curvature in such a way that the rate of mass transport by surface diffusion is nearly independent of curvature, but with a higher activation energy of ~1.01 eV. The curvature dependencies of the apparent surface diffusivity and concentration of mobile surface atoms is believed to be related to the expected lifetime of a mobile surface atom, and has the practical consequence that a simulation study that does not account for this finite lifetime could underestimate the activation energy for mass transport via surface diffusion by ~0.27 eV. 

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3. Linking the Surface and Subsurface in River Deltas—Part 1: Relating Surface and Subsurface Geometries

   https://doi.org/10.1029/2020WR029282  

   Hariharan, Jayaram; Xu, Zhongyuan; Michael, Holly A.; Paola, Chris; Steel, Elisabeth; Passalacqua, Paola (August 2021, Water Resources Research)
4. Linking the Surface and Subsurface in River Deltas - Part 1: Relating Surface and Subsurface Geometries

   Hariharan, J.; Michael, H.A.; Paola, C.; Steel, E.; Passalacqua, P. (January 2021, Water resources research)
5. Fluid Surface Mechanics in Flow and Transport

   https://doi.org/10.36934/TR2022_151  

   Tally, Caroline (June 2022, Williams College Special Collections)

   Fluid surface mechanics are essential to many flow and transport processes. In this thesis, we explore two experimental systems governed by the physics of fluid-surface interactions. First, we consider the evolution of a small fluid leak. We perform simple experiments characterizing the leakage of fluid from a small circular hole in a vertical tube and use real-time and high- speed video to capture transitions between different flow and no-flow regimes, including spontaneous arrest. We create phase diagrams to describe the flow transitions of a leak for varied contact angle, viscosity and hole size. Combining our high-speed video data with an energetic analysis, we find that a leak can stop itself by generating a spherical cap of fluid across the hole that counterbalances the internal pressure driving the leak. However, we find that two conditions must be satisfied for this to occur: first, the potential energy of this “capping droplet” must have a stable minimum given the driving pressure, hole geometry, and material properties; and second, the initial, continuous leak must be unstable against breakup events that lead to generating attempted capping droplets. We also report preliminary results from a new experimental design where we fill the tube from its base with liquid. These filling experiments lead to new flow phase transitions between three leaking behaviors: first, where the liquid level oscillates in the tube; second, where the flow rate out of the hole matches the infusion rate; and third, where the tube overfills. In the second experimental system, we consider the splash transport processes used by Marchantia polymorpha plants. We build a knowledge base of the fundamental physics underlying the fluid-mechanical interactions between these land plants and water, specifically focusing on the splashing mechanics used to facilitate distribution of reproductive material. 

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