Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session S36: Soft Matter at Interfaces: Wetting and Thin FilmsFocus
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Sponsoring Units: GSOFT Chair: Justin Burton, Emory University Room: 339 |
Thursday, March 17, 2016 11:15AM - 11:27AM |
S36.00001: Quantifying stick-slip contact line motion of evaporating sessile droplets Clay Wood, Justin Pye, Justin Burton Sessile droplet evaporation often involves an apparent stick-slip motion of pinning and de-pinning of the drop's edge. The small forces and complex hydrodynamics at the contact line make this phenomena difficult to quantify, although easily observable. We have characterized the stick-slip motion on gold and glass surfaces with the use of a quartz crystal microbalance (QCM). We observe changes in both the resonant frequency and dissipation during droplet evaporation. Depositing a droplet onto this oscillating surface greatly decreases the frequency while the dissipation increases. Evaporation occurs in two stages; when the droplet's contact line is pinned to the surface, its contact angle decreases. Then, at a critical angle, the contact line is pulled over pinning points and continues to evaporate with a receding contact area. These stick-slip events appear in our data as a sharp increase in frequency, followed by a sharp decrease; simultaneously, the dissipation displays a single sharp peak. QCMs pre-cleaned in an oxygen plasma environment exhibited a significantly reduced occurrence and magnitude of these features. We interpret these features and quantify the forces involved in the stick-slip motion using a dynamic model of the QCM with additional surface forces at the contact line. [Preview Abstract] |
Thursday, March 17, 2016 11:27AM - 11:39AM |
S36.00002: High-precision measurements of molecular slip at a solid/liquid interface Justin Pye, Clay Wood, Justin Burton As fluidic devices get smaller and measurements become more precise and stringent, the need to fully understand the dynamics at interfaces becomes more important. It is now clear that slip near an interface is common at the nanoscale in Newtonian liquids. In simple systems, there is a general trend to larger slip lengths for non-wetting liquid/solid combinations, but many conflicting measurements and interpretations remain. We have developed a novel differential technique using a quartz crystal microbalance (QCM) to measure slip lengths on various substrates. A drop of one liquid is grown on the QCM in the presence of a second, ambient liquid. By choosing the two liquids such that their bulk effects on the QCM frequency and dissipation are identical in the presence of no-slip, we are able to isolate anomalous boundary effects due to interfacial slip. Our data for water on gold (in undecane) are consistent with a slip length of 5nm (for water). A glass surface, wetted by both gold and undecane has also shown strongly anomalous results for the water-undecane pair. In addition to investigating other liquid pairs, future work will include extending this technique to surfaces with independently controllable chemistry and roughness, both of which are known to strongly affect interfacial hydrodynamics. [Preview Abstract] |
Thursday, March 17, 2016 11:39AM - 11:51AM |
S36.00003: Asymmetric and speed-dependent contact angle hysteresis and relaxation of a suddenly stopped moving contact line Dongshi Guan, Yong Jian Wang, Elisabeth Charlaix, Penger Tong We report direct atomic-force-microscope measurements of capillary force hysteresis and relaxation of a circular moving contact line (CL) formed on a long micron-sized hydrophobic fiber intersecting a water-air interface. The measured capillary force hysteresis and CL relaxation show a strong asymmetric speed dependence in the advancing and receding directions. A unified model based on force-assisted barrier-crossing is utilized to find the underlying energy barrier Eb and size $\lambda $ associated with the defects on the fiber surface. The experiment demonstrates that the pinning (relaxation) and depinning dynamics of the CL can be described by a common microscopic frame-work, and the advancing and receding CLs are influenced by two different sets of relatively wetting and non-wetting defects on the fiber surface. [Preview Abstract] |
Thursday, March 17, 2016 11:51AM - 12:03PM |
S36.00004: Visualizing the shape of soft solid and fluid contacts between two surfaces Jonathan Pham, Frank Schellenberger, Michael Kappl, Doris Vollmer, Hans-J{\"u}rgen Butt The soft contact between two surfaces is fundamentally interesting for soft materials and fluid mechanics and relevant for friction and wear. The deformation of soft solid interfaces has received much interest because it interestingly reveals similarities to fluid wetting. We present an experimental route towards visualizing the three-dimensional contact geometry of either liquid-solid (i.e., oil and glass) or solid-solid (i.e., elastomer and glass) interfaces using a home-built combination of confocal microscopy and atomic force microscopy. We monitor the shape of a fluid capillary bridge and the depth of indentation in 3D while simultaneously measuring the force. In agreement with theoretical predictions, the height of the capillary bridge depends on the interfacial tensions. By using a slowly evaporating solvent, we quantify the temporal evolution of the capillary bridge and visualized the influence of pinning points on its shape. The position dependence of the advancing and receding contact angle along the three-phase contact line, particle-liquid-air, is resolved. Extending our system, we explore the contact deformation of soft solids where elasticity, in addition to surface tension, becomes an important factor. [Preview Abstract] |
Thursday, March 17, 2016 12:03PM - 12:15PM |
S36.00005: Imbibition kinetics of spherical aggregates Pascal Hébraud, Didier Lootens, Alban Debacker The imbibition kinetics of a millimeter-sized aggregate of 300 nm diameter colloidal particles by a wetting pure solvent is studied. Three successive regimes are observed : in the first one, the imbibition proceeds by compressing the air inside the aggregate. Then, the solvent stops when the pressure of the compressed air is equal to the Laplace pressure at the meniscus of the wetting solvent in the porous aggregate. The interface is pinned and the aggregate slowly degases, up to a point where the pressure of the entrapped air stops decreasing and is controlled by the Laplace pressure of small bubbles. Depending on the curvature of the bubble, the system may then be in an unstable state. The imbibition then starts again, but with an inner pressure in equilibrium with these bubbles. This last stage leads to the complete infiltration of the aggregate. [Preview Abstract] |
(Author Not Attending)
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S36.00006: Tracking single particles motion in shaken wet powder clusters Jennifer Wenzl, Guenter K. Auernhammer, Laurent Gilson In many industrial branches wet granulate powders, where the particles are connected via an additional binding liquid, are widely used. Amply investigated were model systems, where the binding liquid is homogeneously distributed, i.e. building a connecting capillary network. In contrast wet granulate model systems with an inhomogeneous liquid distribution have been rarely in focus of research. In this work a model system for wet powders was developed, which is suitable for 3D imaging with confocal microscopy. Fluorescent silica particles were immersed in a mixture of two immiscible liquids, one continuous and one binding liquid. In detail a wet powder cluster, where the binding liquid formed droplets was studied in 3D. During applying a mechanical load the motion of the powder particles and the binding liquid droplets was followed. Deformation of the binding liquid droplets led to an increase of its surface area and energy. When the droplet relaxed to an energetically more favored shape upon further cluster deformation, the sudden release of the stored surface energy led to complex powder particle and droplet motions. The model system illustrated the complex dynamics upon shaking, and showed that the binding liquid dominated the cluster dynamics on a local scale. [Preview Abstract] |
Thursday, March 17, 2016 12:27PM - 12:39PM |
S36.00007: Force vs. extension of colloidal membranes Leroy Jia, Robert Pelcovits, Thomas Powers, Mark Zakhary, Zvonimir Dogic In experiments, disk-shaped colloidal membranes composed of long rod-like viruses will take on a twisted ribbon shape under the application of a diametric stretching force. We use an effective model valid for membranes with small twist penetration to study this phase transition and calculate the force necessary to stretch the membrane to a given extension. The model predicts that for small deformations, the force is linear with spring constant depending on the effective edge bending stiffness of the membrane, while for large extensions, the force is found to saturate to a constant value. Surprisingly, the force is not a monotonic function of the extension. Finally, we use simple numerical calculations to find a power law that accurately describes the critical stretch at which the membrane starts to twist, which may be used to estimate the value of unknown parameters by comparison with experimental data. [Preview Abstract] |
Thursday, March 17, 2016 12:39PM - 12:51PM |
S36.00008: Thermal-mechanical behavior of self-assembled nanoparticle membranes Yifan Wang, Sean McBride, Xiao-Min Lin, Heinrich Jaeger Monolayers composed of colloidal nanoparticles with a thickness of less than ten nanometers have remarkable mechanical strength and can suspend over micron-sized holes to form free-standing membranes. However, previous measurements on mechanical properties of these monolayers were typically carried out at room temperature. Here, we report the first systematic experimental study of the stiffness of free-standing nanoparticle membranes as a function of temperature. At room temperature and below, these membranes exhibit reversible changes in stiffness, which increases with temperature. At higher temperatures irreversible membrane relaxation was found. This work provides a better understanding of the sub-nanometer scale ligand interactions in self-assembled nanoparticle membranes, and opens up opportunities for using these membranes as thermal-mechanical devices. [Preview Abstract] |
Thursday, March 17, 2016 12:51PM - 1:03PM |
S36.00009: Direct observation of critical adsorption on colloidal particles Hongyu Guo, Zhiyuan Wang, C. E. Bertrand, Paul Douglas Godfrin, Yun Liu We report our direct measurements of critical adsorption on the surface of small spherical silica particles suspended in a binary mixture of lutidine and water, using small-angle neutron scattering (SANS). The surface concentration profile and excess adsorption are studied as functions of temperature, lutidine concentrations, and surface curvature. The profile shape agree with scaling laws. The adsorption associated with the profile shape is found to increase monotonically with increasing lutidine concentration and to decrease with increasing temperature. These observations are important to understand colloidal aggregation behaviors close to the critical point of a binary solvent. [Preview Abstract] |
Thursday, March 17, 2016 1:03PM - 1:15PM |
S36.00010: Obtaining self-similar scalings in focusing flows Joshua Dijksman, Shomeek Mukhopadhyay, Cameron Gaebler, Thomas Witelski, Robert Behringer The surface structure of converging thin fluid films displays self-similar behavior, as was shown in the work by Diez et al [Q. Appl. Math 210, 155, 1990]. Extracting the related similarity scaling exponents from either numerical or experimental data is non-trivial. Here we provide two such methods. We apply them to experimental and numerical data on converging fluid films driven by both surface tension and gravitational forcing. In the limit of pure gravitational driving, we recover Diez' semi-analytic result, but our methods also allow us to explore the entire regime of mixed capillary and gravitational driving, up to entirely surface tension driven flows. We find scaling forms of smoothly varying exponents up to surprisingly small Bond numbers. Our experimental results are in reasonable agreement with our numerical simulations, which confirm theoretically obtained relations between the scaling exponents. [Preview Abstract] |
Thursday, March 17, 2016 1:15PM - 1:27PM |
S36.00011: Visualizing Nanoscopic Topography and Patterns in Freely Standing Thin Films Vivek Sharma, Yiran Zhang, Subinuer Yilixiati Thin liquid films containing micelles, nanoparticles, polyelectrolyte-surfactant complexes and smectic liquid crystals undergo thinning in a discontinuous, step-wise fashion. The discontinuous jumps in thickness are often characterized by quantifying changes in the intensity of reflected monochromatic light, modulated by thin film interference from a region of interest. Stratifying thin films exhibit a mosaic pattern in reflected white light microscopy, attributed to the coexistence of domains with various thicknesses, separated by steps. Using Interferometry Digital Imaging Optical Microscopy (IDIOM) protocols developed in the course of this study, we spatially resolve for the first time, the landscape of stratifying freely standing thin films. We distinguish nanoscopic rims, mesas and craters, and follow their emergence and growth. In particular, for thin films containing micelles of sodium dodecyl sulfate (SDS), these topological features involve discontinuous, thickness transitions with concentration-dependent steps of 5-25 nm. These non-flat features result from oscillatory, periodic, supramolecular structural forces that arise in confined fluids, and arise due to complex coupling of hydrodynamic and thermodynamic effects at the nanoscale. [Preview Abstract] |
Thursday, March 17, 2016 1:27PM - 1:39PM |
S36.00012: Out-of-contact elastohydrodynamic deformation due to lubrication forces. Yumo Wang, Charles Dhong, Joelle Frechette We characterize the spatiotemporal deformation of an elastic film during the radial drainage of fluid from a narrowing gap. Elastic deformation of the film takes the form of a dimple and prevents full contact to be reached. With thinner elastic film the stress becomes increasingly supported by the underlying rigid substrate, the dimple formation is suppressed, which allows the surfaces to reach full contact. We highlight the lag due to viscoelasticity on the surface profiles, and that for a given fluid film thickness deformation leads to stronger hydrodynamic forces than for rigid surfaces. [Preview Abstract] |
Thursday, March 17, 2016 1:39PM - 1:51PM |
S36.00013: Approaching a flat boundary with a block copolymer coated emulsion drop: late stage drainage dynamics. Damith Rozairo, Andrew Croll Understanding the dynamics of the formation and drainage of the thin fluid film that becomes trapped by a deformable droplet as it approaches another object is crucial to the advancement of many industrial and biomedical applications. Adding amphiphilic diblock copolymers, which are becoming more commonly used in drug delivery and oil recovery, only add to the complexity. Despite their increased use, little is known about how long polymer chains fill an emulsion drop’s interface or how the molecules influence hydrodynamic processes. We study the drainage dynamics of a thin water film trapped between mica and a diblock copolymer saturated oil droplet. Specifically, we examine several different polystyrene-b-poly(ethylene oxide) (PS-PEO) molecules self-assembled at a toluene-water interface using laser scanning confocal microscopy. Our experiments reveal that the molecular details of the polymer chains deeply influence the drainage times, indicating that they are not acting as a ‘simple’ surfactant. The presence of the chains creates a much slower dynamic as fluid is forced to drain through an effective polymer brush, the brush itself determined by chain packing at the interface. We present a simple model which accounts for the basic physics of the interface. [Preview Abstract] |
Thursday, March 17, 2016 1:51PM - 2:03PM |
S36.00014: Soft Levelling: Capillary Relaxation of a Thin Liquid Film on an Elastic Substrate Marco Rivetti, Christine Linne, Thomas Salez, Maxence Arutkin, Elie Raphael, Oliver Baeumchen A thin liquid film with non-zero curvature at its free surface spontaneously relaxes towards a flat configuration. The flow of this liquid film is driven by Laplace pressure gradients and it is resisted by viscosity. In the last few years the dynamics of this system has been studied experimentally, numerically and analytically. Inspired by recent progresses on the wetting behaviour of liquid droplets on soft substrates, we here consider the relaxation of a thin viscous film supported by an elastic foundation. We present experiments involving thin polystyrene films on polydimethylsiloxane substrates, where the dynamics of the liquid-air interface is monitored using an atomic force microscope. In this system, Laplace pressure gradients not only drive the flow but they also induce elastic deformations on the substrate. These deformations affect the flow and the shape of the liquid-air interface itself, giving rise to an original example of elasto-capillary interaction that is not mediated by the presence of a contact line. We show that the width of the profile scales with the time to the power 1/6, rather than 1/4 which has been observed on rigid substrates. A theoretical model that describes the coupled evolution of the elastic-liquid and liquid-air interfaces is also presented. [Preview Abstract] |
Thursday, March 17, 2016 2:03PM - 2:15PM |
S36.00015: Slowing of Dynamics of Hydration Water Depends on Length Scale of Measurement Jonathan Nickels, John Atkinson, Souleymane Diallo, Stefania Perticaroli, John Katsaras, John Dutcher The dynamics of hydration water associated with biomolecules is often slower than in bulk. We have used quasielastic neutron scattering (QENS) to study the dynamics of hydration water associated with soft colloidal, monodisperse phytoglycogen nanoparticles. The large water content of the phytoglycogen nanoparticles makes this an ideal system for investigations of hydration water in hydrophilic environments. We find that the hydration water translation is sub-diffusive, occurring, on average, $\sim$ 5.8 times slower than that of bulk water. Significantly, these data demonstrate a clear \textit{q}-dependence in the measured retardation factor, implying a corresponding length scale dependence. This observation may help to reconcile the often-conflicting range of hydration water retardation factors reported in the literature using different experimental techniques. [Preview Abstract] |
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