Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session F53: Fluid Mechanics for Soft Matter II: Soft Interfaces |
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Sponsoring Units: GSOFT GSNP DFD Chair: Jonathan Singer, Rutgers Univ Room: LACC 513 |
Tuesday, March 6, 2018 11:15AM - 11:27AM |
F53.00001: Elongation of vortex street in soap films Ildoo Kim In flowing soap films, it is known that the vorticity field is correlated to the thickness field and that the thickness variation propagates at the Marangoni elastic wave speed. In this circumstance, we modify the point vortex model in the regime where the system's characteristic speed U is comparable to the velocity field propagation speed c in the medium. We show that the the stability condition of von Kármán vortex streets, h/l=0.28, where h and l are the transverse and longitudinal spacing between vortices, scales with 1/(1 + U/c). This result suggests that the vortex streets may look elongated when U/c≈1. |
Tuesday, March 6, 2018 11:27AM - 11:39AM |
F53.00002: Axisymmetric spreading of surfactant on the surface of a deep pool from a point source Shreyas Mandre Guided by computation, we theoretically calculate the steady flow driven by Marangoni stress due to surfactant introduced on a fluid interface at a constant rate. Two separate extreme cases, where the surfactant dynamics is dominated by the adsorbed phase or the dissolved phase are considered. We focus on the case where the size of the surfactant source is much smaller than the size of the fluid domain, and the resulting Marangoni stress overwhelms viscous forces so that the flow is strongest in a boundary layer close to the interface. We derive the resulting flow in a region much larger than the surfactant source but smaller than the domain size by approximating it with a self-similar profile. The radially outward component of fluid velocity decays with the radial distance r as r-3/5 when the surfactant spreads in an adsorbed phase, and as r-1 when it spreads in a dissolved phase. Universal flow profiles that are independent of the system parameters emerge in both the cases. Three hydrodynamic signatures in terms of similarity exponents, the self-similar profiles, and the surface shear stress are identified to distinguish between the two cases and verify the applicability of our analysis with successive stringent tests. |
Tuesday, March 6, 2018 11:39AM - 11:51AM |
F53.00003: Hydrodynamic signatures of stationary Marangoni-driven surfactant spreading Mahesh Bandi, Sathish Akella, Dhiraj Singh, Ravi Singh, Shreyas Mandre Surfactant spreading at air-water interfaces is driven by flow setup by surface tension gradients (Marangoni stress) established by surfactants themselves. We experimentally probe the nature of steady surfactant transport on the interface when the resulting flow is strongest in a thin boundary layer near the interface. In particular, we present three experimental hydrodynamic signatures to distinguish between two limiting cases, viz. adsorption versus dissolution dominated transport, without invoking the surfactant's physico-chemical properties. In a region much larger than the surfactant source, but much smaller than the interfacial area, the steady-state fluid velocity assumes a self-similar form whose magnitude decays as a power-law with the distance from the source. We experimentally demonstrate that this power-law possesses an exponent -3/5 in adsorption and -1 in dissolution dominated flow. Explicit measurement of boundary layer and shear stress provide additional hydrodynamic signatures of surfactant transport mechanisms in the two limiting cases. We test this criterion against two known surfactants, Sodium Dodecyl Sulfate and Tergitol 15-S-9, and apply the results to camphoric acid, with unknown surface properties. |
Tuesday, March 6, 2018 11:51AM - 12:03PM |
F53.00004: Capillary formation of alcohol meniscus and dynamic measurement of its viscoelasticity Wonho Jhe, Dohyun Kim, Dongha Shin, Sangmin An We present a novel method for producing a nanometric meniscus of several alcohols, such as acetone, ethanol and methanol, and the in situ measurement of their mechanical properties such as the viscoelasticity. The atomic force microscope based on the quartz tuning-fork sensor is employed to form and manipulate the nanometric alcohol meniscus, and the theoretical tool of amplitude-modulation atomic force microscopy is used to obtain the elasticity, viscosity and dissipation energy of it. With high vertical resolution less than 0.1 nm and high force sensitivity of 0.01 N/m, this tool facilitates the stable formation and manipulation of a nano-alcohol cluster (~10^4 molecules) in air without ‘jump-to-contact’ instability, as well as quantitative measurements of its physico-chemical properties. |
Tuesday, March 6, 2018 12:03PM - 12:15PM |
F53.00005: The Contaminating Effects of the One Percent Asher Mouat, Clay Wood, Justin Pye, Justin Burton When a volatile liquid drop is placed on a wetting surface, it rapidly spreads and forms a circular, thin film before evaporating. Surprisingly, when even trace amounts (~0.05%) of some less volatile liquid contaminant are present in the volatile liquid, the contact line destabilizes, leading to the formation of structures resembling “fingers” or “pearls”. We have characterized this phenomena using isopropanol with various contaminants (ethylene glycol, glycerol, dodecane, water, and acetone) on surfaces of varying wettability. Although pearls always form due to enhanced evaporation at the contact line, we find that fingers only form when the contaminant has a higher surface tension than the isopropanol, and partially wets the surface. The characteristic size of the structures increases with contaminant concentration. In addition, we find that the local vapor pressure of the isopropanol strongly affects the wetting characteristics of the contaminant. Once the isopropanol evaporates, striking droplet patterns of the contaminant are left behind. Some resemble crystalline lattices of various drop sizes, or the contaminant liquid may form a sub-micron thin film. These highlight the role of trace impurities in volatile liquids, and the deposition patterns they leave behind. |
Tuesday, March 6, 2018 12:15PM - 12:27PM |
F53.00006: Soft-matter morphodynamics: from interfacial instabilities to elastic structures Pierre-Thomas Brun, Lingzhi Cai, Joel Marthelot The surge of modern techniques to fabricate structured materials poses great challenges to existing manufacturing paradigms. Fluid mechanics has the potential to dramatically impact this segment, particularly through the directed control of interfacial instabilities. We show that the destabilization of a liquid interface into a pattern forming instability may be harnessed to fabricate complicated hierarchical and topological soft elastic structures. We use elastomeric mixtures that are initially liquid and cure in finite time thereby freezing the instability-mediated structures. We study the morphodynamics of this process, that is the link between the pattern growth and its morphology. We propose a possible pathway to solve the inverse problem: designing the optimal set of initial conditions and interactions that will be transmuted into a target shape without direct external intervention. |
Tuesday, March 6, 2018 12:27PM - 12:39PM |
F53.00007: Thin Film Patterning: an Electrostatics-to-Hydrodynamics Inverse Problem and Its Solution Chengzhe Zhou, Sandra Troian Micro- and nanopatterning techniques for applications ranging from optoelectronics to biofluidics have multiplied in number over the past decade to include adaptations of mature technologies as well as novel lithographic techniques based on periodic spatial modulation of surface stresses. In this talk we focus on shape deformation of nanofilms under the presence of a patterned counter-electrode. Induced polarization charges at the liquid interface causes a patterned electrostatic pressure counterbalanced by surface tension which leads to 3D protrusions whose evolution can be terminated as needed. We formulate a nonlinear evolution equation of liquid film shape governing the electrohydrodynamic response under a preset counter-electrode pattern from variational principle. The corresponding inverse problem of finding designs of counter-electrode pattern that drive film into desired 3D structures is solved by determining optimal control for the nonlinear dynamic system. Optimality conditions are derived and an efficient numerical algorithm is presented. We demonstrate such implementation of film control to achieve periodic, free surface shapes ranging from simple arrays of Gaussian bumps to more complex sawtooth and ring patterns. |
Tuesday, March 6, 2018 12:39PM - 12:51PM |
F53.00008: Effect of an external field on capillary waves in a dipolar fluid Jason Koski, Stan Moore, Gary Grest, Mark Stevens A well-known aspect of liquid/vapor systems is the presence of capillary waves at an interface. Capillary waves are density fluctuations that result of a balance between thermal fluctuations and the surface tension. The role of an external field on capillary waves at the liquid-vapor interface a dipolar fluid is investigated using molecular dynamics simulations. For fields parallel to the interface, the interfacial width squared increases linearly with respect to the logarithm of the size of the interface across all field strengths tested. The value of the slope decreases with increasing field strength indicating that the field dampens the capillary waves. With the inclusion of the parallel field, the surface stiffness increases with increasing field strength faster than the surface tension. For fields perpendicular to the interface, the interfacial width squared is linear with respect to the logarithm of the size of the interface for small field strengths and the surface stiffness is less than the surface tension. Above a critical field strength that decreases as the size of the interface increases, the interface becomes unstable due to the increased amplitude of the capillary waves. |
Tuesday, March 6, 2018 12:51PM - 1:03PM |
F53.00009: In Situ Rheology of Complex Fluids Under High Pressure Jonathan Dench, Neal Morgan, Janet Wong Complex fluids are ubiquitous in our everyday life. In some biological and engineering processes, very thin fluid films are present and often they experience high pressure and shear. Such fluids include polymer or micellar solutions and mixtures of polymer/oligomers. Their rheology across different length scales, from the nanometre to the microscale, is difficult to resolve using conventional techniques when there are spatial heterogeneities. Fluorescence anisotropy and molecular probes of differing sizes provide the opportunity to study the local physics of such fluids, under high pressures and shear stresses. This work presents the local rheology of a complex fluid system under high shear stress, using fluorescence anisotropy measurements, at pressures up to 1 GPa. The effects of fluid composition and shear stress are investigated. |
Tuesday, March 6, 2018 1:03PM - 1:15PM |
F53.00010: Hygrobot: A bio-inspired walking device driven by environmental humidity energy Beomjune Shin, Jonghyun Ha, Kyu-Jin Cho, Ho-Young Kim Micro-robots that are light and agile yet requiring no artificial power input can be widely used in medical, military, and industrial applications. Here we report an actuation system that propels itself by harnessing the environmental humidity energy with its motion rectified by a ratchet. The bilayer-type actuator employs a hygroscopically responsive layer consisting of aligned nanofibers as inspired by the awns of self-burrowing seeds of Erodium and Pelargonium species. The ratchets based on asymmetric friction coefficients rectify the oscillatory bending motion into a directional locomotion like those found in inchworms, snails and snakes. We mathematically analyze the mechanical response of the actuator-ratchet system, the hygrobot, by solving the vapor diffusion and elastic bending of the active layer simultaneously, which allows us to optimize the robot design for maximum locomotion speed. We further demonstrate the utility of the humidity driven system for some biomedical applications. |
Tuesday, March 6, 2018 1:15PM - 1:27PM |
F53.00011: Elastic hoops jumping on water: A model system for fishing spiders Han Bi Jeong, Eunjin Yang, Yunsuk Jeung, Juliette Amauger, Ho-Young Kim Small semiaquatic arthropods such as water striders and fishing spiders have amazing maneuverability including jumping on water. The physical principle behind the jump of fishing spiders is discriminated from that of water striders in that the spiders use primarily the water drag, instead of the surface tension, to launch themselves into air. Here we present a simple physical model of the fishing spider’s jump by studying the jump of an elastic hoop off the water surface. An initially compressed hoop that floats on water is set free by removing a constraint, so that the elastic strain energy is converted into the kinetic energy. The hoop pushes the water surface at a similar dynamic condition to fishing spiders. The dynamic interaction of an elastically vibrating hoop and deforming water surface determines the take-off velocity of the hoop, which eventually gives the maximum jump height. Our theory considering both hoop vibration and free-surface flow of water is in good agreement with experiment, revealing that the jump efficiency, defined as the ratio of the hoop’s kinetic energy at take-off to the initial strain energy is approximately only 6%. Our theory can be used to find the optimal design of water jumpers utilizing drag forces. |
Tuesday, March 6, 2018 1:27PM - 1:39PM |
F53.00012: Particle pair dynamics in rheologically complex interfaces Harishankar Manikantan, Todd Squires The surface viscosities of many insoluble surfactants can vary dramatically under conditions accessible to rather mundane flows, and can give rise to a host of qualitatively new phenomena. In particular, surface-pressure-dependent surface rheology can impact interfacial microhydrodynamics in significant ways. We study particle pair dynamics in thin interfacial gaps (using lubrication theory) and in the far-field (using the Lorentz reciprocal theorem) and build up intuition for the behavior of particle pairs in a 2D suspension. We first show that kinematic reversibility of Stokes flows is broken for particles embedded in a surfactant-laden interface when translating or rotating near an interfacial barrier. In the case of particle pairs, the fore-aft symmetry corresponding to a Newtonian interface is lost, leading to well-separated (when pressure-thickening) or aggregated (when pressure-thinning) particles. Based on these irreversible pair interactions, we hypothesize that pressure-thickening (or -thinning) leads to shear-thinning (or -thickening) in 2D suspensions. |
Tuesday, March 6, 2018 1:39PM - 1:51PM |
F53.00013: Janus Colloids Actively Moving at the Surface of Water Antonio Stocco, Maurizio Nobili, Christophe Blanc, Martin In Active motion of living or artificial swimmers (active colloids) may occur in strong confinement and in complex environments. Motion properties of confined active colloids may strongly differ from the ones observed in the liquid bulk. Recently, we showed that the interface between two fluids act as a confinement, which dramatically affects frictions of partially wetted colloidal particles [1]. Here, the impact of partial wetting dynamics on the motion of active Janus colloids will be presented. Immersion depth of the Janus colloids as well as their orientation with respect to the water surface reveal complex and rich wetting properties of Janus particles at the air-water interface [2]. Active directional and circular trajectories have been observed at the interface with a motion persistence significantly enhanced by the partial wetted state of the Janus particles. This confined state impacts the in-plane and out-of-plane particle rotational diffusions, and leads to a truly two dimensional active motion [3]. |
Tuesday, March 6, 2018 1:51PM - 2:03PM |
F53.00014: Ordering pH-Responsive Polyelectrolyte-Grafted Nanoparticles in a Flow Coating Process Chongfeng Zhang, Thomas Carlson, Siyang Yang, Pinar Akcora The effects of nanoscale interparticle interactions on deposition patterns in the flow coating process are investigated using pH-responsive poly(acrylic acid) (PAA)-grafted silica nanoparticles. Interactions between nanoparticles are effectively controlled by grafting densities, PAA brush lengths and pH, in addition to hydrogen bonding between free poly(vinylpyrrolidone) (PVP) and PAA. Consequently, various intriguing patterns of randomly distributed dots, polygonal networks, meshes, fork-like structures along with highly regulated and densely packed stripes parallel to the moving direction of substrates are fabricated. Per se, the flow coating process is shown to form regulated patterns during evaporation by controlling particle-particle interactions with inherent brush properties and external pH. In addition, we will present the rheology results of these functionalized nanoparticles to correlate their structure-viscosity relationships in solution. Hydrogen bonding capability of free poly(vinyl pyrrolidone) (PVP) chains with the grafted PAA is utilized to obtain intra- and inter-particle crosslinking systems. |
Tuesday, March 6, 2018 2:03PM - 2:15PM |
F53.00015: When a colloidal drop evaporates on an inclined surface Jin Young Kim, Najaf Rubab, Hyoungsoo Kim, Byung Mook Weon Colloidal drops including micro- and nanoparticles usually leave ring-like deposit patterns when they evaporate on a flat surface, as well known as the coffee-ring effect. A curious and important geometry is an inclined surface, where colloidal drops may evaporate in different ways with a flat surface. Prediction of final deposit patterns on an inclined surface is inaccessible due to complexity of evaporation processes and profound competition of hydrodynamic and gravitational effects. Here we explore how gravitational force affects evaporation dynamics and final deposit patterns of colloidal drops by varying particle size, particle concentration, and tilting angle. We show and explain how inclination changes evaporation rates and final patterns. This study would shed light on new knowledge for manipulation of external forces on final deposit patterns of colloidal fluids under various situations. |
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