Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session B24: Drops ILive
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Sponsoring Units: DFD Chair: Pedro Saenz, University of North Carolina at Chapel Hill |
Monday, March 15, 2021 11:30AM - 11:42AM Live |
B24.00001: Partial Leidenfrost Evaporation-Assisted Fast Solute Enrichment in a Water Droplet on Hierarchical Micro/Nanostructures Jiangtao Cheng The conventional methods of creating superhydrophobic surface-enhanced Raman spectroscopy (SERS) devices are by conformally coating a nanolayer of hydrophobic materials on micro/nanostructured plasmonic substrates. However, the hydrophobic coating may partially block hot spots and therefore compromise Raman signals of analytes. Here, we report partial Leidenfrost evaporation-assisted fast enrichment of low-concentration analytes in water droplets on hierarchical plasmonic micro/nanostructures, which are fabricated by implanting nanoantennas on carbon nanotubes-decorated Si micropillar arrays. In comparison with natural evaporation, partial Leidenfrost-assisted evaporation provides a levitating force to maintain the analyte droplet in the Cassie-Wenzel hybrid state. In this process, the continuous shrinking circumferential rim of the droplet, which is in the Cassie state, towards the pinned central region of the droplet, which is in the Wenzel state, results in a fast concentration of dilute analyte molecules on a significantly reduced footprint within several minutes. Partial Leidenfrost droplet can reduce the final deposition footprint by 3-4 orders of magnitude and enable ultrasensitive detection of nanomolar analytes. |
Monday, March 15, 2021 11:42AM - 11:54AM Live |
B24.00002: Direct friction force measurements of water droplets rolling on superhydrophobic surfaces Matilda Backholm, Daniel Molpeceres, Maja Vuckovac, Heikki Nurmi, Matti Hokkanen, Ville Jokinen, Jaakko Timonen, Robin Ras Extremely water repellent surfaces are common in nature and widely mimicked for technological applications such as anti-wetting, anti-fouling, and anti-fogging coatings. As the quality of these artificial superhydrophobic surfaces reach new, improved levels, the existing characterisation techniques of their slipperiness have become inadequate. In our work, we have applied micropipette force sensors to directly quantify droplet friction forces at nanonewton resolution (Backholm, et al., Commun. Mater. 1, 64 (2020)). We have studied the physics and fluid dynamics of water droplets moving on extremely slippery superhydrophobic surfaces and report a surprising rolling motion of slowly moving droplets. Our findings enable the search for even more slippery surfaces as the wetting community approaches the extreme limit of slipperiness. |
Monday, March 15, 2021 11:54AM - 12:06PM Live |
B24.00003: A granular analogue of the Plateau-Rayleigh instability in bubble rafts Carmen Lee, Kari Dalnoki-Veress Granular materials are useful as analogues to molecular systems due to their breadth of easily tuneable properties. In particular, a granular material may be described by continuum mechanics as the number of the particles transitions from few to many. For example, a raft of bubbles can exhibit dual properties of a granular system and a liquid system. Here, we examine the interplay between surface tension and buoyancy in the breakup of a raft of bubbles that is initially shaped into a thin strip. This breakup of a strip into nearly-circular domains is a 2D analogue of the Plateau-Rayleigh Instability breakup of a liquid jet into droplets that is observed in 3D. Bubbles are held at the surface of a water bath in a strip, which is suddenly released resulting in a breakup into regularly spaced, nearly-circular clusters of bubbles. Bubble radius and the width of the bubble raft strip are tuned to investigate different aspects of the 2D “granular Plateau-Rayleigh Instability”, including satellite clusters, and late stage coarsening. |
Monday, March 15, 2021 12:06PM - 12:18PM Live |
B24.00004: Time evolution of colloidal stratification in a drying droplet Zhiyu Jiang, Chong Shen, Lanfang Li, Ye Xu, H Daniel Ou-Yang, Carly Roche In a droplet drying, uniformly mixed colloidal particles with two differ sizes could separate either due to the difference in particle diffusion or the convection flow induced by difference in buoyancy. The role for each mechanism causing the stratification is unclear if we only watch the final distribution of particles after drying. To distinguish how each mechanism stratifies the particles, we need to control diffusiophoresis and convection by varying size ratio of big to small particles and the degree of density mismatching respectively. We record time evolution of distributions for two-sized particles to understand how particles are stratified by each mechanism. The experiments are designed into a droplet drying process confined between two parallel plates where fluorescent particles are tracked by a confocal fluorescence microscopy and analyzed by MATLAB imaging processing. The results show the two-type particles accumulate at different locations when the direction of the convection flow changes. This talk will present the spatiotemporal evolution of the concentration for small and big particles and show the stratification can be controlled by adjusting the convection and diffusiophoresis. |
Monday, March 15, 2021 12:18PM - 12:30PM Live |
B24.00005: Impact of surface viscosity on the stability of a droplet translating through a stagnant fluid. Natasha Singh, Vivek Narsimhan This study examines the impact of interfacial viscosity on the stability of an initially deformed droplet translating through an unbounded quiescent fluid. The boundary-integral formulation is employed to investigate the time evolution of the droplet in the Stokes flow limit. The viscous droplet interface is modeled using the Boussinesq-Scriven constitutive relationship. We observe that below a critical value of the capillary number, CaC, the initially perturbed droplet reverts to its spherical shape. Above this CaC, the translating droplet deforms continuously, resulting in a growing tail at the rear end for initial prolate perturbations and a cavity for initial oblate perturbations. We find that the presence of surface shear viscosity inhibits the tail/cavity growth at the droplet's rear end and increases the CaC compared to a clean droplet. In contrast, surface dilational viscosity increases tail/cavity growth and lowers CaC compared to a clean droplet. We explore the mechanisms behind each of these observations in our talk and provide phase diagrams for how these interfacial effects alter the critical capillary number for different values of the droplet's viscosity ratio and initial Taylor deformation parameter. |
Monday, March 15, 2021 12:30PM - 12:42PM Live |
B24.00006: Coalescence speed of two equal-sized nanobubbles Eric Bird, Jun Zhou, Zhi Liang We use molecular dynamics (MD) simulations coupled with continuum-based theoretical analysis to study the coalescence dynamics of two equal-sized nanobubbles (NBs). By varying the bubble diameter, we change the NB Ohnesorge number from 0.46 to 0.33 and directly measure the bridge radius, rb, as a function of time, t. In all cases, we find the prediction from the axisymmetric Navier-Stokes equation overestimates the expansion speed of the capillary bridge at early time of NB coalescence. However, once we take into account the curvature-dependent surface tension and restrict the minimum principal radius at the capillary bridge to the size of the atom in the model liquid, the theoretical prediction agrees with the MD data very well in both early time and later time of the coalescence process. From the theoretical model, we find neither liquid viscous force nor liquid inertial force dominates at later time of coalescence of the model NBs. In this case, the MD simulation results show rb(t)∝t0.76±0.04 with scaling exponent considerably higher than that in the scaling law rb(t)∝t0.5 for the viscous and inertial dominated regimes. |
Monday, March 15, 2021 12:42PM - 12:54PM Live |
B24.00007: Drop impact of anisotropic colloidal suspensions Phalguni Shah, Srishti Arora, Ravi Chepuri, Michelle R Driscoll Fluid drop impact has been an active field of study for more than a century, but the impact of complex fluid drops is a relatively young field. The impact of colloidal drops at high impact velocities allows us to probe suspension flow behavior at shear rates larger than existing rheometers can access. Recently, suspension drops of spherical particles have been observed to exhibit jamming at high impact velocities. In order to explore the effect of anisotropy on impact dynamics, we experimentally study the drop impact of rod-shaped colloidal suspensions. We prepare and use micron-sized silica rods of multiple aspect ratios, thus treating aspect ratio as another parameter in the state diagram of suspension behavior, in addition to impact velocity and suspension volume fraction. Our observations suggest that impacting drops of rod-shaped suspensions show thickening and jamming at lower volume fractions as compared to sphere suspensions, which is in agreement with recent rheological studies of rod-shaped particle suspensions. Furthermore, the extent of jamming and elastic behavior is more pronounced for higher impact velocities and aspect ratios. |
Monday, March 15, 2021 12:54PM - 1:06PM Live |
B24.00008: Quasi-two-dimensional coalescence of liquid in another liquid Yukina Koga, Ko Okumura
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Monday, March 15, 2021 1:06PM - 1:18PM On Demand |
B24.00009: Parametric Charactrization of Coaxial Drop Dripping Encapsulation Process Ahmed Ismail, Jose Rafael Castrejon-Pita, Helena Azevedo Encapsulation of one compound to another is a process that shapes our lives in many aspects. For instance, drug delivery, nutraceuticals and children’s food with minerals and vitamins production, among many other applications, all depend on the encapsulation process. The desired features for an effective encapsulation process include capsule size control, encapsulating the right amount of the core liquid, and ensuring that the capsule external wall strength is suitable for the application. In this study, we present a detailed characterization on encapsulation via coaxial drop dripping into a liquid bath. The liquid bath contains an ionic solution capable of inducing the gelation of the drop external liquid consisting of a viscous solution made of a polymer of opposite charge. A parametric mapping of the successful encapsulation conditions is established, this include the inner and outer flow rates and the viscosity of the external liquid. Simple relations are produced to predict the amount of liquid encapsulated and the size of the capsules. We observe that the concentration of the external liquid solution is one of the key controlling parameters, which influence the strength of the capsule after drying, along with the wall thickness and the capsule size. |
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