Session P48: Thin Films, Surface Flows, Interfaces and Microfluidics II

Characteristic interfacial structure behind a rapidly moving contact line

A solid rapidly pushed into a liquid, of viscosity η, at velocity U entrains surrounding air along with its moving surface. Using high-speed interferometry we find a robust characteristic structure of the entrained air layer: there is a thin-thick alternation of gap thickness in the transverse direction and this feature occurs both in wetting and de-wetting. In the thin regions we find that the gap thickness scales approximately as (ηU)^0.5. We present a model using the assumption that the velocity profile is robust to thickness fluctuations that gives a good quantitative estimate of this gap thickness. This is in contrast to the Landau-Levich analysis which had previously been assumed applicable to this problem.



  

Levitation of fizzy fluids

Liquids generally spread on solids they encounter. However, under particular circumstances, such a wetting can be reduced or even avoided, a situation of obvious practical interest in terms of anti-adhesion or even thermal and mechanical insulations. By using superhydrophobic coating, a water drop can thus limit its contact with the substrate at a low fraction. Liquid levitation can even be achieved using the volatility of the liquid and creating a thin insulating vapor layer as early described by Leidenfrost in 1756. More recently, other strategies have been developed involving external fields such as air flows, Faraday waves or magnetic fields. We here describe a novel approach using active liquids able to sustain levitation in the absence of external forces. We will first discuss the levitation of carbonated water. In this regime, the drop self-generates the gas cushion which provides levitation. We will model the lubrication theory behind this insulation. Finally, we will generalize this new regime to different kinds of substrates and non-volatile liquids.   

Dewetting Front Instabilities for Micro-patterning

It has long been known that fluid instabilities can be harnessed for low-effort self-assembly of ordered structures on the nano- and micro- scales. Here, we demonstrate how a known thin film instability resulting from Van der Waals forces in an evaporating film can generate a number of extraordinarily ordered nano- and micro-structures. The patterns that can arise include hexagonal lattices, lines, branches, and triangular sawtooth structures. We find that the patterning mechanism is driven by fluid dynamics and Ostwald ripening of crystallizing salts at the fluid dewetting front. Controllability of these patterns can be further enhanced by application of flow-control strategies. We present a phase diagram of substrate properties that result in patterning, and perform stability analysis to predict the wavelength of the instability. Such patterns may have potential applications in sensor arrays, photonics, dielectric materials, and materials of controlled porosity.   

Nanosecond Pulsed Imaging for Ethanole Electrospraying Breakup in a Modified Nozzle

An electrified liquid jet breakup behavior and modes of disintegration indifferent ow rates and applied voltages are investigated. The present phenomenology belongs to a new injector introduced recently by Morad et al. (2016). This injector is shown to highly extend the stability and ow rate of electrospray particularly in the Taylor cone-jet mode. The experimental investigation is performed using a high-power light-emitting diode (LED) illumination as the light source. The light source is developed to operate in the pulsing condition when synchronized with a digital camera, and is particularly designed to function properly in presence of high electromagnetic interference (EMI). Details of the light source development is presented with the captured images from the jet breakup at different modes. The operational map of the new injector is obtained, and the physical mechanisms of different behaviors are explained and discussed.   

From droplets to waves: interfacial instabilities of viscosity-stratified flows in microchannels

Rapid layering of viscous materials in microsystems encompasses a range of hydrodynamic instabilities that facilitate mixing and emulsification processes of fluids having large differences in viscosity. We experimentally study dispersed droplet and separated viscous wave flow regimes arising from viscosity-stratified microflows made of fluid pairs with large viscosity ratios and systematically investigate the effects of control parameters such as flow rate, viscosity ratio, and interfacial tension between model fluid pairs. We demonstrate that key features of periodic droplets and interfacial viscous waves, including emission frequency, propagating celerity, and wavelength, can be readily described by functional relationships, which delineate effects of inertial, capillary and viscous forces. We also shed light on wave crest breaking process, which produces viscous ligaments that continuously transport thick material into the fast co-flowing low-viscosity stream. Finally, we examine the transition from droplet to wave regime to provide a comprehensive scenario of interfacial instabilities in microfluidic viscosity-stratified flows.
  

High precision characterization of a binary fluid interface using surface light scattering spectroscopy

Thermally excited capillary waves (ripplons) with an rms height of ~1 nm perturb any fluid interface. The Doppler spectrum of these ripplons, which can be characterized by Surface Light Scattering Spectroscopy (SLSS), depend upon interfacial properties such as surface tension and surface viscoelasticity, via the capillary wave surface response function, a refinement of the classical dispersion equation. Innovative optical design has increased signal and signal-to-noise ratio. This enhances measurement accuracy over the entire range of wave numbers, while enabling measurements at higher wave numbers above 1500/cm. We demonstrate the validity of the technique with high precision and high accuracy measurements of standard fluids. Subsequent measurements of pentane/2-methyl pentane mixtures anticipate an upcoming NASA microgravity experiment intended to optimize the effectiveness of wickless heat pipes for space flight applications.   

NEMD Simulations Quantifying the Influence of Surfaces Roughness on Conductive Heat Transfer at Liquid/Solid Interfaces

Rapid developments in extreme ultraviolet lithography are soon expected to produce integrated circuit components with feature sizes of about 10 nm. At these small scales, the interfacial or so-called Kapitza resistance, which tends to increase with diminishing system size, progressively hinders the transfer and evacuation of heat, ultimately compromising performance. Although numerous experiments and simulations over the past few decades have elicited trends in the Kapitza resistance which correlate with system size and surface roughness, good quantitative understanding for atomistically rough interfaces is still lacking. In this talk, we discuss some of the effects on Kapitza resistance induced by surface roughness at liquid/solid interfaces as investigated by non-equilibrium molecular dynamics (NEMD) simulations using realistic solid walls modeled by an interacting 12-6 Lennard-Jones potential. Comparison between atomistically smooth and rough interfaces, contrasting such behavior as the vibrational density of states, helps interrelate the phonon spectrum and phonon modes transmitted across the interface with various quantitative measures characterizing interfacial roughness.   

Oblique water entry of micron particles

Water entry problem has attracted much interest in the past century, but few research focused on the oblique water entry of micron particles, which is very common in nature and industrial processes. The oblique water entry of micron PMMA particles with different impact angles is realized by a self-made impactor and observed with a high speed camera. It was found that the distortion of the liquid surface after impact is non-axisymmetric due to the lateral velocity of the particle, and the wetted part of the particle surface is not axisymmetric along the impact direction, which produces a force perpendicular to the impact direction and makes the particle’s motion direction changes during the water entry process. The critical velocity for the particle to sink increases with the decrease in impact angle. A simple model is established to analyze the energy balance during the critical sinking process and the expression of the critical sinking velocity is given, which agrees well with our experimental results.   

Rebound dynamics of a superhydrophobic sphere

Small solid particles impacting a water surface can become trapped, rebound, or pass through the interface [Lee & Kim, Langmuir, 24 (2008)]. In the present work, we focus on the bouncing dynamics of millimetric superhydrophobic spheres impacting the surface of a quiescent water bath. Through experiments, we characterize the dependence of the normal coefficient of restitution and contact time on the impact velocity, radius, and relative density of the sphere. We also highlight a qualitatively new regime identified that occurs near the sinking threshold for many spheres. Ongoing work and future directions will be discussed.   

Characterization of Liquid Transport in MicroCapillaries via X-Ray Photon Correlation Spectroscopy

X-ray photon correlation spectroscopy (XPCS), a novel x-ray scattering technique analogous to dynamic light scattering, enables probing the hydrodynamics of liquids on length scales ranging from microns to nanometers and time scales ranging from seconds to microseconds. The National Synchrotron Light Source II (NSLS-II) of Brookhaven National Laboratory (BNL), which started operating in 2015 as one of the world's most advanced synchrotrons can enable XPCS studies with unprecedented spatio-temporal resolution. This presentation will discuss ongoing work at the Coherent Hard X-ray Scattering (CHX) beamline of NSLS-II where XPCS techniques are employed to characterize flow velocity profiles and mass diffusivity in microcapillaries. A genetic algorithm is developed to determine local flow velocities and diffusion coefficients from the intensity autocorrelation function obtained from XPCS experiments. The developed XPCS techniques and analytical methods can provide new insights into the hydrodynamic and rheological behavior of liquids in micro/nanoscale confinement.   

Suppression of the viscous fingering instability by shear

During the viscous fingering instability, the interface between two fluids confined to a thin gap is unstable to finger formation when the less viscous fluid invades the higher viscoity one. Here, we study the instability between pairs of miscible fluids. When experiments are conducted in a radial geometry there is a period of stable growth which persists up to a critical radius that depends on the viscosity ratio of the fluids and the injection rate. The presence of fingers appears to be tied to specific interfacial structure across the small dimension of the gap. In this work we see how actively interfering with this structure formation affects the instability. By applying an oscillatory translational shear to the top plate of our Hele-Shaw cell we observe a dramatic delay in the onset of the instability. We suggest that this delay is tied to changing the curvature of the interface at the leading edge of the pattern. This phenomenon could lead to novel control techniques to suppress this instability.   

Experimental Pressure Analysis Techniques in the Viscous Fingering Instability

When a lower viscosity fluid displaces a higher viscosity one in a confined geometry, the interface between the two fluids is unstable to the formation of fingers. The fluid flow is governed by Darcy’s law so that the local velocity is proportional to the pressure gradient. Using thin rings of dyed fluid we visualize the local flow field within a radial Hele-Shaw cell and are able to map pressure gradients throughout the pattern-formation process. In particular, we measure the pressure gradients as a function of distance from the unstable fluid interface. Using various image processing techniques, we examine the role of the fluids’ viscosity ratio as a parameter governing pattern formation.

  

Equilibrium shapes and their stability for liquid films in fast flows

We present the results of a combined experimental and theoretical investigation of a suspended liquid film deformed by an external flow en route to forming a bubble. We identify a family of nonminimal but stable equilibrium shapes for flow speeds up to a critical value, beyond which the film inflates unstably. A model based on free-streamline theory accounts for the observed nonlinear deformations and forces. Our theoretical predictions suggest that bubble formation at low speeds results from the instability of overly-inflated shapes, and at high speeds from the loss of equilibrium solutions.