Session J50: Emulsions and Foams

Iridescent Patterns and Flows in Vertical Foam Films

Liquid foams consist of bubbles separated by thin films. Individual films consist of two surfactant-laden surfaces that are $\sim$ 5 nm -- 10 micron apart. Sandwiched between these interfacial layers is a fluid that drains primarily under the influence of gravitational, viscous and interfacial forces, including disjoining pressure. Understanding and controlling the drainage kinetics of thin films is an important problem that underlies the stability, lifetime and rheology of foams and emulsions. We experimentally follow the drainage kinetics of foam films using imaging {\&} color science. Interference between light reflected from two surfactant-laden surfaces that are $\sim$ 100 nm - 10 micron apart leads to thickness-dependent iridescent colors in the visible region. Below 50 nm the thin films appear as black. We find fascinating examples of two-dimensional hydrodynamics and unexplained, if not unprecedented, drainage kinetics. In particular, we study the origin of marginal regeneration, i. e. the complex flow patterns that originate near the borders of foam films.   

Stratifying Foam Films and Micelle Aggregation Number

The shelf-life, stability and rheology of liquid foams depends upon the processes that drive drainage and rupture in thin liquid films. Foam films containing micelles, colloidal particles or polyelectrolyte-surfactant mixtures exhibit step-wise thinning or stratification, often attributed to the formation of ordered structures and the layer-by-layer removal of them. Using a Scheludko-type cell, we experimentally study the stratification kinetics of horizontal foam films formed by aqueous sodium dodecyl sulfate (SDS) solutions, and carefully determine how the concentration of surfactant influences the stepwise thinning process. We elucidate how quantitative characterization of stratification provides a method for measuring dynamic disjoining pressure, as well as for estimating micelle size and interactions. The concentration-dependent aggregation number, and micelle charge extracted from our experiments match-up reasonably well with values obtained by other techniques including scattering and fluorescence.   

Capillary foams: highly stable bubbles formed by synergistic action of particles and immiscible liquid

Liquid foams are a familiar part of everyday life from beer and frothed milk to bubble baths; they also play important roles in enhanced oil recovery, lightweight packaging, and insulation. We report a new class of foams, obtained by frothing a suspension of colloidal particles in the presence of a small amount of an immiscible secondary liquid. A unique aspect of the new foams, termed capillary foams, is that suspended particles mediate spreading of a minority liquid around gas bubbles. The resulting mixed particle/liquid coating can stabilize bubbles against coalescence even when the particles alone cannot. We demonstrate the generality of capillary foams by forming them from a diverse set of particle/liquid combinations and rationalize the results with a simple free energy model. In addition to many applications as liquid foams, capillary foams can serve as precursors for hierarchically-structured solids with porosity on different length scales and with significant application potential.   

Hyperuniformity Length in Experimental Foam and Simulated Point Patterns

Systems without long-wavelength number density fluctuations are called hyperuniform (HU).~The degree to which a point pattern is HU may be tested in terms of the variance in the number of points inside randomly placed boxes of side length L. If HU then the variance is due solely to fluctuations near the boundary rather than throughout the entire volume of the box. To make this concrete we introduce a hyperuniformity length h, equal to the width of the boundary where number fluctuations occur. ~Thus h helps characterize the disorder. ~We show how to deduce h from the number variance,~and we do so for Poisson and Einstein patterns plus those made by the vertices and bubble centroids in 2d foams. A Poisson pattern is one where points are totally random. These are not~HU and~h equals L/2. We coin~``Einstein patterns'' to be where points in a lattice are independently displaced from their site~by a normally distributed amount. These are~HU and h equals the RMS displacement from the lattice sites. Bubble centroids and vertices are both HU. For these, h is less~than L/2 and increases slower than linear in L. The centroids are more HU than the vertices, in that h that increases more slowly.   

The Response of a 2D Emulsion to Local Perturbations

We experimentally perturb a quasi-two-dimensional emulsion packing by inflating an oil droplet into the system in a controlled way. Our samples are oil-in-water emulsion confined between two close-spaced parallel plates, so that the droplets are deformed into pancake shapes. In this system, there is only viscous friction and no static friction between droplets. By imaging the droplets with a video microscopy, we observe rearrangement events induced by the local perturbation. Simultaneously, we measure droplet-droplet contact forces by analyzing the outlines of each droplet in our movies. These allow us to study how the packings with varying degrees of spatial order have different responses to the local perturbation.   

Simulations of Soft Glassy Matter with Ripening

Soft glassy matter (SGM) such as foams, emulsions, and colloids, exhibit interesting rheological properties that have long defied explanation. In particular, the shear modulus of these materials displays weak power law frequency dependence. To understand the origin of this property in more depth, we have built a three-dimensional, modified Bubble Dynamics model. The bubbles interact with a purely repulsive harmonic potential and ripen according to diffusion-based governing equations. An energy minimizer is implemented to quasi-statically relax topological rearrangements in the system as ripening proceeds. Preliminary results show that the model displays expected intermittent particle rearrangements and a weakly frequency-dependent shear modulus behaving like a power law fluid. We find that the anomalous relaxation properties and avalanche-like nature of the rearrangements can be related to different measures of the systems potential energy landscape.   

Arresting relaxation in Pickering Emulsions

Pickering emulsions consist of droplets of one fluid dispersed in a host fluid and stabilized by colloidal particles absorbed at the fluid-fluid interface. Everyday materials such as crude oil and food products like salad dressing are examples of these materials. Particles can stabilize non spherical droplet shapes in these emulsions through the following sequence: first, an isolated droplet is deformed, e.g. by an electric field, increasing the surface area above the equilibrium value; additional particles are then adsorbed to the interface reducing the surface tension. The droplet is then allowed to relax toward a sphere. If more particles were adsorbed than can be accommodated by the surface area of the spherical ground state, relaxation of the droplet is arrested at some non-spherical shape. Because the energetic cost of removing adsorbed colloids exceeds the interfacial driving force, these configurations can remain stable over long timescales. In this presentation, we present a computational study of the ordering present in anisotropic droplets produced through the mechanism of arrested relaxation and discuss the interplay between the geometry of the droplet, the dynamical process that produced it, and the structure of the defects observed.   

Suppression of Ostwald Ripening by Chemical Reactions

Emulsions consisting of droplets immersed in a fluid are typically unstable and coarsen over time. One important coarsening process is Ostwald ripening, which is driven by the surface tension of the droplets. Ostwald ripening must thus be suppressed to stabilize emulsions, e.g. to control the properties of pharmaceuticals, food, or cosmetics. Suppression of Ostwald ripening is also important in biological cells, which contain stable liquid-like compartments, e.g. germ granules, Cajal-bodies, and centrosomes. Such systems are often driven away from equilibrium by chemical reactions and can thus be called active emulsions. Here, we show that non-equilibrium chemical reactions can suppress Ostwald Ripening, leading to stable, monodisperse emulsions. We derive analytical approximations of the typical droplet size, droplet count, and time scale of the dynamics from a coarse-grained description of the droplet dynamics. We also compare these results to numerical simulations of the continuous concentration fields. Generally, we thus show how chemical reactions can be used to stabilize emulsions and to control their properties in technology and nature.   

Membrane-mediated colloidal interactions

Membrane proteins are known to play a key role in inducing curvature in biological membranes. This curvature leads to interactions between the proteins through minimization of the bending energy of the membrane. Simulations have shown a wide variety of interesting phenomena, for example how curvature influences protein sorting, but little is understood about the underlying physics. We study these membrane-mediated interactions using an experimental model system consisting of micron-sized polymer particles linked to a freestanding lipid membrane (GUV). The particles locally distort the membrane curvature and by that exhibit an attraction. The influence of the curvature distortion on the pairwise interaction is studied systematically by tracking the particles with confocal microscopy.   

Contact-line deformation around a spherical particle at an anisotropic liquid interface

The shape of the contact line around a particle determines its interaction with other particles at liquid interfaces. Thus, the shape of the interface and contact line is significant for self-assembly and many other applications of colloids. In our experiments, we used PDMS-coated millimeter-sized glass spheres to avoid pinning. The contact line around the sphere is observed at initially flat, cylindrical-like and saddle-like shapes with a camera placed perpendicular to the plane of the initially flat interface. Unlike flat interfaces, at anisotropic interfaces, the contact line around the sphere is not circular. Our results demonstrate that the quadrupolar deformation of the contact line (z$_{2})$ increases with deviatoric curvature (anisotropy) of the interface ($D_{0})$. For instance, for a PDMS-coated glass sphere with a diameter 3.2mm, as $D_{0}$ increases from 0 to 0.12mm$^{-1}$, z$_{2}$ increases from 0 to about 0.3mm. We will discuss the relation among z$_{2}$, $D_{0}$, mean contact radius, particle radius, and contact angle and compare to theory. Our results are important to understand the assembly of particles at anisotropically curved interfaces. This work is funded by the NSF through CBET-0967620 and by the Gulf of Mexico Research Initiative through the C-MEDS consortium.   

The influence of protein aggregation on adsorption kinetics

When proteins adsorb to an air-water interface they lower the surface tension and may form an age-dependent viscoelastic film. Protein adsorption to surfaces is relevant to both commercial uses and biological function. The rate at which the surface tension decreases depends strongly on temperature, solution pH, and protein structure. These kinetics also depend on the degree to which the protein is aggregated in solution. Here we explore these differences using Chymotrypsinogen as a model protein whose degree of aggregation is adjusted through controlled heat treatment and measured by chromatography. To study these effects we have used a micropipette tensiometer to produce a spherical-cap bubble whose interfacial pressure was controlled -- either steady or oscillating. Short heat treatment produced small soluble aggregates, and these adsorbed faster than the original protein monomer. Longer heat treatment produced somewhat larger soluble aggregates which adsorbed more slowly. These results point to complex interactions during protein adsorption.   

Gravitational Drainage of Anionic and Nonionic Surfactant Mixtures for Foamability Enhancement

Two surfactant mixture solutions were used to study gravitational drainage from thin vertical planar films and in a settler column. The surfactant mixtures contained anionic surfactant Sodium Dodecyl Sulfate (SDS) and nonionic superspreader trisiloxane SILWET L-77 at different mixing ratios. We found the relation between the lifetime of planar vertical film and foamability of the surfactant solution. Namely, solutions with longer lifetimes in planar film drainage reveal a higher foamability. Also, the foamability of the mixed surfactant systems was found to be greater than the foamability of each of the individual components.   

ZrP nanoplates based fire-fighting foams stabilizer

Firefighting foam, as a significant innovation in fire protection, greatly facilitates extinguishments for liquid pool fire. Recently, with developments in LNG industry, high-expansion firefighting foams are also used for extinguishing LNG fire or mitigating LNG leakage. Foam stabilizer, an ingredient in fire-fighting foam, stabilizes foam bubbles and maintains desired foam volume. Conventional foam stabilizers are organic molecules. In this work, we developed a inorganic based ZrP (Zr(HPO4)2$\cdot$H2O, Zirconium phosphate) plates functionalized as firefighting foam stabilizer, improving firefighting foam performance under harsh conditions. Several tests were conducted to illustrate performance. The mechanism for the foam stabilization is also proposed.   

Maximum bubble pressure tensiometry and foamability

The stability of a freshly created foam is intimately linked with the rate of mass transfer of a surfactant from liquid sub-phase to the interface, and this diffusion- or adsorption-limited kinetics is said to impact the so-called foamability. The time dependent variation in surface tension can also become a factor in controlling response to dilatational deformations, as kinetic effects due to mass transfer also enter into the description of Gibbs-Marangoni elasticity of surfaces. Dynamic surface tension measurements carried out with conventional methods like pendant drop analysis, Wilhelmy plate, etc are limited in their temporal resolution (\textgreater 50 ms). In this study, we describe design and application of maximum bubble pressure tensiometry for the measurement of dynamic surface tension effects at extremely short (1-50 ms) timescales. We discuss the ramifications of this nearly unprecedented capability for unraveling physics underlying high speed printing and foaming with small molecule surfactant solutions.   

Janus nanoparticles for stable microemulsions with ultra-low IFT values

Janus particles are an influential type of materials used in foams, detergents, surfactants and cosmetics. Due to their demonstrated flexibility and non-toxicity, they have the potential to replace molecular surfactants, and thanks to their amphiphilicity, they can stabilize immiscible biphasic systems. Disk-based Janus particles best perform this stabilization. Graphene has been used to manufacture this class of particles; however, their fabrication in high yield by short and atomically economic syntheses remains a challenge. In this project we report the first synthesis of monolayer disks by a one pot reaction under microwave energy. Using a scalable method, these disks were synthesized, emulsified (in an oil/water system), and chemically reacted to obtain the Janus nanodisks with an efficient method. Our nanosheets production technique is a promising approach for the fabrication of Janus nanodisks via emulsification as it produces IFT (interfacial tension) values in a lower range than that of the molecular surfactants. These ultra-low values, in conjunction with the sheets' salt resistance, temperature resistance, and non-toxicity position Janus particles as the next generation of nanosurfactants.