Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session D25: Focus Session: Hydrodynamics of Particles and Macromolecules at Fluid Interfaces II |
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Chair: Carlos E. Colosqui, Stony Brook University Room: Georgia World Congress Center B313 |
Sunday, November 18, 2018 2:30PM - 2:43PM |
D25.00001: Formation, Development and Disruption of a Particle Coating on a Confined Bubble Charles Sharkey, Zixian Cui, Shelley Anna Surface-active nanoparticles adsorb at fluid interfaces, imparting elasticity and stability against coalescence. Nanoparticles diffuse relatively slowly, desorb very slowly, and interact colloidally, allowing the coverage and mechanical properties of the interface to be manipulated in a fluid flow. We examine the coupling of hydrodynamics and particle adsorption for elongated bubbles translating along a capillary tube filled with surface-active nanoparticle suspension. Particle adsorption times are comparable to bubble residence times, and are controlled via flow rate, tube length, and suspension viscosity. Particles are convected to the trailing end of the bubble, yielding a rigid trailing cap. Short bubbles exhibit a uniform thickness Bretherton film. For longer bubbles the leading end thin film sharply transitions to a thick film at the trailing end, characteristic of a Marangoni stress on the interface. At higher speeds, the trailing cap appears to crumple, shedding layers of nanoparticles. We relate these observations to the interfacial hydrodynamics and the development of elasticity on the bubble interface. These observations highlight the critical role of processing history on formation of particle-stabilized interfaces. |
Sunday, November 18, 2018 2:43PM - 2:56PM |
D25.00002: Microparticles hydrodynamics close to an Air-Water Interface Stefano Villa, Antonio Stocco, Christophe Blanc, Maurizio Nobili Despite being by far among the most widespread interface on Earth, with a pervasive presence in environment, biology, industry and daily life, physical phenomena at Air-Water Interface (AWI) are still largely elusive. This fact is related to the complexity of AWI with its peculiar electrostatic and flow boundary conditions. In this paper, we present a study of the dynamics and interactions of charged microparticles diffusing at submicrometric distances from an AWI. An interferometric set-up mounted on a microscope gives access to an highly resolved trajectory of the colloid (5-10 nm). Contrary to other established techniques (like TIRM and holography) our technique has the great advantage of allowing the measure of the absolute particle-interface distance. In order to control the electrostatic we use different concentration of salt and different pH in association with pH dependent charged microparticles. Surprisingly new equilibrium distances from the AWI are measured not accounted by established theories. In addition different drags corresponding to different boundaries conditions are found for same combination of particle and AWI depending on the particle’s motion direction namely perpendicular or parallel to the AWI. |
Sunday, November 18, 2018 2:56PM - 3:09PM |
D25.00003: Squirmers adhered to fluid interfaces Nicholas G Chisholm, Kathleen Joan Stebe We theoretically investigate active colloids adhered to a clean fluid-fluid interface of arbitrary viscosity ratio in Stokes flow. We adopt the squirmer model in which a spherical colloid is made to self-propel by assigning a tangential slip velocity to its surface. We assume that the fluid maintains a 90° contact angle at the three-phase contact line with the colloid, the contact line is fixed, and the capillary number is small. Thus, we assume a flat fluid-fluid interface bisecting the squirmer at its meridian, and the squirmer is only allowed to translate and rotate in the interfacial plane. Unlike a self-propelled colloid in the bulk, a swimmer attached to an interface may exert a net force and/or torque on the fluid, which are balanced by capillary forces on the swimmer itself. Thus, in the far field, the flow is dominated by a stresslet that can be aligned at an arbitrary angle to the interface. As a result, squirmers that push (pull) against the interface create purely attractive (repulsive) interactions with neighboring bodies. Additionally, by including rotational modes, two adhered squirmers can orbit about a common center or exhibit cooperative self-propulsion. |
Sunday, November 18, 2018 3:09PM - 3:22PM |
D25.00004: Free energy landscape of Janus nanoparticles at a liquid/vapor interface Joel Koplik, Charles Maldarelli While homogeneous colloidal particles select their position at an interface based on density and equilibrium contact angle alone, surface heterogeneities can produce multiple local equilibria with varying immersion depths and particle orientation, and hysteretic behavior in dynamic situations. The understanding and potential control of colloidal configurations is facilitated by the free energy landscape – the particle’s free energy as a function of its immersion depth and orientation – which is easily calculated in the continuum approximation assuming a flat liquid/vapor interface. Using molecular dynamics simulations for a Janus nanoparticle, we calculate the molecular-level free energy landscape by thermodynamic integration, incorporating both thermal fluctuations and bending of the interface due to local wetting angles on the particle surface. We find significant differences between continuum and molecular free energies surfaces both in numerical values and in the presence of local minima, and illustrate the distinction by simulations of particle migration to the interface from either the vapor or the liquid phase. |
Sunday, November 18, 2018 3:22PM - 3:35PM |
D25.00005: Adsorption and denaturation of structured polymeric nanoparticles at an interface Jie Feng, Chang Tian, H. Jeremy Cho, Sujit S Datta, Robert K Prud'homme Nanoparticles (NPs) have been widely applied in fields as diverse as energy, environment, and human health. However, the adsorption and trapping of NPs at interfaces is still poorly understood. In many applications, such as drug delivery, understanding NP interactions at an interface is essential to determine and control adsorption onto targeted areas. Therapeutic NPs are especially interesting because their structures involve somewhat hydrophilic surface coronas, to prevent protein adsorption, and much more hydrophobic core phases. Here, we investigate the evolution of NP attachment and structural evolution at the air–liquid interface over time scales from 100 ms to 10s of seconds. We document three distinct stages in NP adsorption. In addition to an early stage of free diffusion and a later one with steric adsorption barriers, we find a hitherto unrealized region where the interfacial energy changes due to surface “denaturation” or restructuring of the NPs at the interface. We adopt a quantitative model to calculate the diffusion coefficient, adsorption rate and barrier, and extent of NP hydrophobic core exposure at different stages. Our results deepen the fundamental understanding of adsorption of structured NPs at an interface. |
Sunday, November 18, 2018 3:35PM - 3:48PM |
D25.00006: The dynamics of flexible and Brownian filaments in viscous flows Yanan Liu, Brato Chakrabarti, David Saintillan, Anke Lindner, Olivia Du Roure The morphological dynamics, instabilities and transitions of elastic filaments in viscous flows underlie a wealth of biophysical processes, and are also key to deciphering the rheological behavior of many complex fluids and soft materials. Here, we combine experiments with actin filaments as an experimental model system and computational modeling with non-local slender body theory to elucidate the dynamical regimes and morphological transitions of elastic Brownian filaments in a simple shear flow. The filament dynamics and transitions are primarily governed by a dimensionless elasto-viscous number comparing viscous drag forces to elastic bending forces, with thermal fluctuations only playing a secondary role. The transitions are characterized by a buckling instability and the appearance of localized high-curvature bends that propagate along the filaments in apparent “snaking” motions. We present a complete characterization of filament morphologies and transitions as a function of elasto-viscous number and scaled persistence length and demonstrate excellent agreement between theory, experiments and simulations. |
Sunday, November 18, 2018 3:48PM - 4:01PM |
D25.00007: PIV measurement of flow induced on a salt waterbody by a freshwater source Islam Benouaguef1, Edison C Amah, Naga A Musunuri, Denis Blackmore, Ian Fischer, Pushpendra Singh The particle image velocimetry (PIV) technique is used to study the flow induced on a salt waterbody when a freshwater source is present on the surface. The measurements show that the source causes a strong axisymmetric solutocapillary flow about the vertical line passing through the source. The fluid directly below the source rises upward, and near the surface it moves away. The PIV data is also compared with the analytical solution of this problem. |
Sunday, November 18, 2018 4:01PM - 4:14PM |
D25.00008: Inertial Marangoni Propulsion: Experiments Samrat Sur, Hassan Masoud, Jonathan Philip Rothstein In this presentation, the experimentally observed Marangoni-driven motion of disk-shaped and spherical particles located at an air-water interface atop a deep layer of water will be presented. The particle self-propulsion is due to the release of a surface active agent (here alcohol or surfactants) from its rear which locally reduces the surface tension. The Marangoni surfer is expected to undergo a pure translation. However, at a high enough Reynold numbers, the left-right symmetry was broken giving rise to rotation of the particle. After the initial rotation, the particle was found to follow a nearly circular path. Particle tracking and PIV measurements were used to understand the origin of the rotational motion. When the strength of the propulsion by changing the concentration of alcohol released, a critical Reynolds number of approximately Re = 200 was discovered beyond which an asymmetric vortex shedding was observed in the wake of the disks and spheres. The asymmetric vortex shedding results in a periodic torque on the particle that can result in the observed particle rotation. In the limit of vanishingly small Reynolds numbers, particle rotation can also be induced through an interaction with interface curvature near the edges of the dish. |
Sunday, November 18, 2018 4:14PM - 4:27PM |
D25.00009: Inertial Marangoni Propulsion: Simulation Saeed Jafari Kang, Esmaeil Dehdashti, Jonathan Philip Rothstein, Hassan Masoud We study the Marangoni-driven motion of a disk-shaped particle located at a flat air-water interface sitting atop a deep layer of water. The particle self-propels thanks to the release of an active agent from its rear that locally reduces the surface tension. The release is set such that the resulting surface tension gradients produce no rotating torque about the direction of propulsion if the motion is straight. Hence, provided that outer boundaries are symmetric and no external perturbations are present, the Marangoni surfer is expected to undergo a pure translation. However, we have found experimentally that, at high enough Reynold numbers, the left-right symmetry is broken spontaneously, which gives rise to the rotation of the particle. After the initial rotation, the particle follows a nearly circular path until it runs out of the "fuel" or gets close to the boundaries. Here, we numerically examine whether a perturbation in the lateral velocity of the particle can lead to its spontaneous and self-sustained circular motion. Such a perturbation may lead to the asymmetric shedding of vortical structures that appear on the sides of the particle at high speeds. The asymmetric vortex shedding could then be responsible for the particle rotation and its subsequent trajectory. |
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