Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session G28: Surface Tension Effects: Particles at Interfaces |
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Chair: Shreyas Mandre, Brown University Room: 309 |
Monday, November 23, 2015 8:00AM - 8:13AM |
G28.00001: Hydrodynamics of a fixed camphor boat at the air-water interface Dhiraj Singh, Sathish Akella, Ravi Singh, Shreyas Mandre, Mahesh Bandi A camphor tablet, when introduced at the air-water interface undergoes sublimation and the camphor vapour spreads radially outwards across the surface. This radial spreading of camphor is due to Marangoni forces setup by the camphor concentration gradient. We report experiments on the hydrodynamics of this process for a camphor tablet held fixed at the air-water interface. During the initial transient, the time-dependent spread radius $R(t)$ of camphor scales algebraically with time $t$ ($R(t) \propto t^{1/2}$) in agreement with empirical scalings reported for spreading of volatile oils on water surface. But unlike surfactants, the camphor stops spreading when the influx of camphor from the tablet onto the air-water interface is balanced by the outflux of camphor due to evaporation, and a steady-state condition is reached. The spreading camphor however, shears the underlying fluid and sets up bulk convective flow. We explain the coupled steady-state dynamics between the interfacial camphor spreading and bulk convective flow with a boundary layer approximation, supported by experimental evidence. [Preview Abstract] |
Monday, November 23, 2015 8:13AM - 8:26AM |
G28.00002: Hydrodynamics of a self-propelled camphor boat at the air-water interface Sathish Akella, Dhiraj Singh, Ravi Singh, Mahesh Bandi A camphor tablet, when placed at the air-water interface undergoes sublimation and camphor vapour spreads radially outwards across the surface due to Marangoni forces. This steady camphor influx from tablet onto the air-water interface is balanced by the camphor outflux due to evaporation. When spontaneous fluctuations in evaporation break the axial symmetry of Marangoni force acting radially outwards, the camphor tablet is propelled like a boat along the water surface. We report experiments on the hydrodynamics of a self-propelled camphor boat at air-water interfaces. We observe three different modes of motion, namely continuous, harmonic and periodic, due to the volatile nature of camphor. We explain these modes in terms of ratio of two time-scales: the time-scale over which viscous forces are dominant over the Marangoni forces ($\tau_{\eta}$) and the time-scale over which Marangoni forces are dominant over the viscous forces ($\tau_{\sigma}$). The continuous, harmonic and periodic motions are observed when $\tau_{\eta}/\tau_{\sigma} \sim 1$, $\tau_{\eta}/\tau_{\sigma} \geq 1$ and $\tau_{\eta}/\tau_{\sigma} \gg 1$ respectively. Experimentally, the ratio of the time scales is varied by changing the interfacial tension of the air-water interface using Sodium Dodecyl Sulfate. [Preview Abstract] |
Monday, November 23, 2015 8:26AM - 8:39AM |
G28.00003: PIV measurements of the streaming fluid flow due to the adsorption of particles Harsh Patel, Naga Musunuri, Edison Amah, Ian Fischer, Pushpendra Singh The particle image velocimetry (PIV) technique is used to study the streaming flow that is induced when a spherical particle is adsorbed at a liquid surface. The flow causes powders sprinkled on a liquid surface to disperse on the surface. The dispersion can occur so quickly that it appears explosive, especially for small particles on the surface of mobile liquids like water. The measurements show that the adsorption of a spherical particle causes an axisymmetric streaming flow about the vertical line passing through the center of the particle. The fluid directly below the particle rises upward, and near the surface, it moves away from the particle. The flow, which develops within a fraction of second after the adsorption of the particle, persists for several seconds. The flow strength, and the volume over which it extends, decreases with decreasing particle size. The streaming flow induced by the adsorption of two or more particles is a combination of the flows which they induce individually. [Preview Abstract] |
Monday, November 23, 2015 8:39AM - 8:52AM |
G28.00004: Particle-laden interfaces: direct calculation of interfacial stress from a discrete particle simulation of a pendant drop Chuan Gu, Lorenzo Botto The adsorption of solid particles to fluid interfaces is exploited in several multiphase flow technologies, and plays a fundamental role in the dynamics of particle-laden drops. A fundamental question is how the particles modify the effective mechanical properties of the interface. Using a fast Eulerian-Lagrangian model for interfacial colloids, we have simulated a pendant drop whose surface is covered with spherical particles having short-range repulsion. The interface curvature induces non-uniform and anisotropic interfacial stresses, which we calculate by an interfacial extension of the Irving-Kirkwood formula. The isotropic component of this stress, related to the effective surface tension, is in good agreement with that calculated by fitting the drop shape to the Young-Laplace equation. The anisotropic component, related to the interfacial shear elasticity, is highly non uniform: small at the drop apex, significant along the drop sides. The reduction in surface tension can be substantial even below maximum surface packing. We illustrate this point by simulating phase-coarsening of a two-phase mixture in which the presence of interfacial particles ``freezes'' the coarsening process, for surface coverage well below maximum packing [Preview Abstract] |
Monday, November 23, 2015 8:52AM - 9:05AM |
G28.00005: Marangoni-Driven Flow Oscillations during the Dissolution of Surfactant Powders Orest Shardt, Hyoungsoo Kim, Hassan Masoud, Howard Stone When particles of surface-active compounds are deposited on a liquid surface, they exhibit a variety of motions, such as the classic erratic movement of camphor on water. We investigate the unsteady motion of a water surface covered with surfactant particles and find that a rapid longitudinal oscillation occurs as the particles dissolve. This phenomenon happens with several common surfactant powders, but it is particularly striking with calcium propionate, an organic salt that decreases the surface tension of water. We examine the effects of several parameters on the characteristics of the oscillation by using particle image velocimetry (PIV). Due to the short period of the oscillation (on the order of 0.1 s) compared to the timescale of surfactant diffusion, we neglect diffusion and model this phenomenon by considering the evolution of variations in surfactant concentration along a liquid surface. This surfactant concentration is advected by the flow that is driven by Marangoni stress due to non-uniform surfactant and therefore surface tension distributions. We examine the critical conditions for and characteristics of the oscillation in this model through theory and simulations. [Preview Abstract] |
Monday, November 23, 2015 9:05AM - 9:18AM |
G28.00006: Evaporation effects in elastocapillary aggregation Dominic Vella, Andreas Hadjittofis, Kiran Singh, John Lister We consider the effect of evaporation on the aggregation of a number of elastic objects due to a liquid's surface tension. In particular, we consider an array of spring--block elements in which the gaps between blocks are filled by thin liquid films that evaporate during the course of an experiment. Using lubrication theory to account for the fluid flow within the gaps, we study the dynamics of aggregation. We find that a non-zero evaporation rate causes the elements to aggregate more quickly and, indeed, to contact within finite time. However, we also show that the number of elements within each cluster decreases as the evaporation rate increases. We explain these results quantitatively by comparison with the corresponding two-body problem and discuss their relevance for controlling pattern formation in carbon nanotube forests. [Preview Abstract] |
Monday, November 23, 2015 9:18AM - 9:31AM |
G28.00007: Elastocapillary-mediated interfacial assembly Arthur Evans Particles confined to an interface are present in a large number of industrial applications and ubiquitous in cellular biophysics. Interactions mediated by the interface, such as capillary effects in the presence of surface tension, give rise to rafts and aggregates whose structure is ultimately determined by geometric characteristics of these adsorbed particles. A common strategy for assembling interfacial structures relies on exploiting these interactions by tuning particle anisotropy, either by constructing rigid particles with heterogeneous wetting properties or fabricating particles that have a naturally anisotropic shape. Less explored, however, is the scenario where the interface causes the particles to deform. In this talk I will discuss the implications for interfacial assembly using elastocapillary-mediated interactions. The competition between surface energy and elasticity can wrinkle and buckle adsorbed soft particles, leading to complicated (but programmable) aggregates. [Preview Abstract] |
Monday, November 23, 2015 9:31AM - 9:44AM |
G28.00008: Stokesian Dynamic Simulations of Colloid Assembly at a Fluid Interface Archit Dani, Charles Maldarelli The collective dynamics and self-assembly of colloids floating at a gas/liquid or a liquid/liquid interface is a balance between deterministic lateral interaction forces, e.g. capillary attraction and dipolar electrostatic repulsion if the particles are charged, viscous resistance to colloid motion along the surface and thermal fluctuations. As the colloid size decreases, thermal (Brownian) forces become important and can affect the self assembly into ordered patterns and crystal structures that are the starting point for materials applications. Stokesian dynamics simulations are presented to describe the lateral organization of particles along the surface in Brownian dominated regimes that includes (using a pairwise approximation) capillary attraction and the hydrodynamic interaction between particles (incorporating the effect of the particle immersion depth) and thermal fluctuations. Clustering, fractal growth and particle ordering are observed at critically large values of the Peclet numbers, while smaller values yield states in which particles remain uncorrelated in space and more widely separated. [Preview Abstract] |
Monday, November 23, 2015 9:44AM - 9:57AM |
G28.00009: Tilting and tumbling of Janus nanoparticles at sheared interfaces Shahab Shojaei-Zadeh, Hossein Rezvantalab We use molecular dynamics simulations to investigate the response of individual Janus nanoparticles adsorbed at liquid-fluid interfaces to imposed symmetric shear. Depending on particle shape, amphiphilicity, and the applied shear rate, two distinct rotational dynamics toward a steady particle orientation are observed: smooth tilting, and tumbling. Particles adopt a steady orientation when imposing shear-induced torque is balanced with the opposing capillary-induced one. The tumbling dynamics can be perhaps explained based on the free energy of different states relative to the thermal energy. We construct phase diagrams correlating particle dynamics to shear rate, hydrophobicity, and shape. Our results have direct implication on flow-induced alignment and assembly of Janus particles at fluid interfaces. [Preview Abstract] |
Monday, November 23, 2015 9:57AM - 10:10AM |
G28.00010: Surface Tension Mediated Under-Water Adhesion of Rigid Spheres on Soft, Charged Surfaces Shayandev Sinha, Siddhartha Das Understanding the phenomenon of surface-tension-mediated under-water adhesion is necessary for studying a plethora of physiological and technical phenomena, such as the uptake of bacteria or nanoparticle by cells, attachment of virus on bacterial surfaces, biofouling on large ocean vessels and marine devices, etc. This adhesion phenomenon becomes highly non-trivial in case the soft surface where the adhesion occurs is also charged. Here we propose a theory for analyzing such an under-water adhesion of a rigid sphere on a soft, charged surface, represented by a grafted polyelectrolyte layer (PEL). We develop a model based on the minimization of free energy that, in addition to considering the elastic and the surface-tension-mediated adhesion energies, also accounts for the PEL electric double layer (EDL) induced electrostatic energies. We show that in the presence of surface charges, adhesion gets enhanced. This can be explained by the fact that the increase in the elastic energy is better balanced by the lowering of the EDL energy associated with the adhesion process. The entire behaviour is further dictated by the surface tension components that govern the adhesion energy. [Preview Abstract] |
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