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1. 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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2. 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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3. 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)
4. 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)

   Abstract River deltas are densely populated regions of the world with vulnerable groundwater reserves. Contamination of these groundwater aquifers via saline water intrusion and pollutant transport is a growing threat due to both anthropogenic and climate changes. The arrangement and composition of subsurface sediment is known to have a significant impact on aquifer contamination; however, developing accurate depictions of the subsurface is challenging. In this work, we explore the relationship between surface and subsurface properties and identify the metrics most sensitive to different forcing conditions. To do so, we simulate river delta evolution with the rule‐based numerical model, DeltaRCM, and test the influence of input sand fraction and steady sea level rise (SLR) on delta evolution. From the model outputs, we measure a variety of surface and subsurface metrics chosen based on their applicability to imagery and modeling results. The Kullback‐Leibler (KL) divergence is then used to quantitatively gauge which metrics are most indicative of the imposed forcings. Both qualitative observations and the KL divergence analysis suggest that estimates of subsurface connectivity can be constrained using surface information. In particular, more variable shoreline roughness values and higher surface wetted fraction values correspond to increased subsurface connectivity. These findings complement traditional methods of estimating subsurface structure in river‐dominated delta systems and represent a step toward the identification of a direct link between surface observations and subsurface form. 

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5. Investigating Surface and Sub-Surface Damage in IM7/8552 via in-situ Synchrotron X-ray Computed Tomography

   https://doi.org/10.2514/6.2020-0248  

   Hanhan, Imad; De Carlo, Francesco; Sangid, Michael (January 2020, AIAA SciTech 2020 Forum)

   Polymer matrix composites are popular in the aerospace industry due to their high strength to weight ratio. While they have become popular, understanding and predicting their specific damage evolution mechanisms remains a challenge especially in designing with damage tolerance criteria. One challenge often faced is the presence of surface damage either induced during manufacturing, machining, or service of a composite part. While many studies have investigated how quasi-static, low-velocity, and ballistic impact results in damage in the material, there remains a need to further understand the reduction in performance that results from such surface damage. In this work, micro-indentation was conducted on a unidirectional IM7/8552 laminate composite specimen to induce quasi-static impact damage that results in surface damage. The specimen was then loaded in tension to 33% of its expected failure load and imaged using synchrotron X-ray micro-computed tomography to qualitatively investigate the progression of surface damage into sub-surface damage. This work shows that at 33% of tensile failure load, surface damage propagates into delamination and fiber breakage of plies directly sub-surface. This work sheds light on the progression of surface damage at loads less than 50% of the ultimate strength of a unidirectional laminate composite. 

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