Session Y45: Focus Session: Soft Matter Physics of Drops, Bubbles, Foams and Emulsions - Bubbles, Films, Foams

Mechanical properties of surface modified microbubbles by Atomic Force Microscopy (AFM)

Atomic force microscopy has been used to investigate the mechanical properties of phospholipid coated microbubbles and to quantify their stiffness. The mechanical properties were investigated using tipless AFM cantilevers to compress microbubbles attached to a gold surface in aqueous conditions. The phospholipid microbubbles were produced by microfluidic flow focusing and were found to have a stiffness of 25 mN/m. The attachment of a streptavidin coating increased the microbubble stiffness by a factor of 30 to around 750 mN/m. Further, the effect of temperature on the mechanical and time dependent properties of bubbles has been studied, the results of which have demonstrated that increasing temperature leads to a decrease in microbubble stiffness and an increase creep-displacement. The standard linear model was used to extract to the visco-elastic parameters at different temperatures, this allowed the first determination of the activation energy for creep for a microbubble.   

Cylindrical bubbles and blobs from a Class II Hydrophobin

\textit{Cerato ulmin} is a class II hydrophobin. In aqueous suspensions, it easily forms cylindrical air bubbles and cylindrical oil blobs. The conditions for formation of these unusual structures will be discussed, along with scattering and microscopic investigations of their remarkable stability. Possible applications in diverse fields including polymer synthesis and oil spill remediation will be considered. Acknowledgment is made to Dr. Wayne C. Richards of the Canadian Forest Service for the gift of \textit{Cerato ulmin}.   

The collapse and the folding of a particle rafts under compression

Compressing a single-layer of particles or bubbles that are confined to the air-water interface results in a range of interesting collapse dynamics. We report on the collapse modes of two systems: (1) a single layer of gas bubbles at the surface and (2) a single layer of polypropylene beads. Under compression, both systems exhibit a critical areal density beyond which there is a transition to a multi-layered structure. Generally speaking, the transformation is characterized by localized submergence into the subphase of bubbles or beads. For both systems, we observe single bubbles/beads being pushed underneath surrounding particles. However, for sufficiently small beads, we observe a folding mode, which corresponds to the long-ranged one dimensional wrinkling of the monolayer surface. In this talk, we will report on the transition between single particle submergence and folding, as well as general characterization of the collapse as a function of compression speed and initial structure of the particle raft.   

Falling drops skating on a film of air

When a raindrop hits a window, the surface immediately becomes wet as the water spreads. Indeed, this common observation of a drop impacting a surface is ubiquitous in our everyday experience. I will show that the impact of a drop on a surface is a much richer, more complex phenomenon than our simple experience may suggests: To completely wet the surface the drop must first expel all the air beneath it; however, this does not happened instantaneously. Instead, a very thin film of air, only a few tens of nanometers thick, remains trapped between the falling drop and the surface as the fluid spreads. The thin film of air serves to lubricate the drop enabling the fluid to skate laterally outward at strikingly high velocities. Simultaneously, the wetting fluid spreads inward at a much slower velocity, trapping a bubble of air within the drop. However, these events occur at diminutive length scales and fleeting time scales; therefore, to visualize them we develop new imaging modalities that are sensitive to the behavior right at the surface and that have time resolution superior to even the very fastest cameras. These imaging techniques reveal that the ultimate wetting of the surface occurs through a completely new mechanism, the breakup of the thin film of air through a spinodal like dewetting process that breaks the cylindrical symmetry of the impact and drives an anomalously rapid spreading of a wetting front. These results are in accord with recent theoretical predictions and challenge the prevailing paradigm in which contact between the liquid and solid occurs immediately, and spreading is dominated by the dynamics of a single contact line.   

Compression of multiwall microbubbles

Optical monitoring of structural transformations and transport processes is prohibited if the objects to be studied are bulky and/or non-transparent. This paper is focused on the development of a microbbuble platform for acoustic imaging of heterogeneous media under harsh environmental conditions including high pressure ($<$500 atm), temperature ($<$100\r{ }C), and salinity ($<$10 wt{\%}). We have studied the compression behavior of gas-filled microbubbles composed of multiple layers of surfactants and stabilizers. Upon hydrostatic compression, these bubbles undergo significant (up to 100$\times )$ changes in volume, which are completely reversible. Under repeated compression/expansion cycles, the pressure-volume P(V) characteristic of these microbubbles deviate from ideal-gas-law predictions. A theoretical model was developed to explain the observed deviations through contributions of shell elasticity and gas effusion. In addition, some of the microbubbles undergo peculiar buckling/smoothing transitions exhibiting intermittent formation of facetted structures, which suggest a solid-like nature of the pressurized shell. Preliminary studies illustrate that these pressure-resistant microbubbles maintain their mechanical stability and acoustic response at pressures greater than 1000 psi.   

The life cycle of individual boiling bubbles: Insights from beyond optical imaging

With a high-speed camera, we have investigated the dynamics of individual vapor bubbles boiling on a laser-heated surface.~ Their sizes and shapes as they grow and depart from a surface are correlated with simultaneous thermal imaging measurements of the boiling surface using thermoreflectance-based microscopy to measure temperatures of individual stochastic events.~ Analysis of both the thermal profiles and the bubble shapes suggests the presence of an evaporating liquid microlayer under the developing bubble.~ Tuning surface and heating properties, we control the shapes of bubbles, ranging from regular periodic growth and departure to stochastic bubbles which exhibit rapid cavitation-like expansion and collapse.~ Unlike typical cavitation bubbles which collapse and form jets pointed towards the surface, jets from bubbles observed during boiling were observed to be directed away from the surface.~ By tuning the wettability of the substrate, we will report on how wettability affects the strength and direction of these jets.   

Nonclassical Thermomigration of an Air Bubble

We study air bubbles confined in capillaries with a temperature gradient. Classically, air bubbles move in a temperature gradient due to decreased surface tension at higher temperatures, creating a net surface traction towards the cold pole, pushing the bubble towards the hot pole for mass conservation. Here we report non-classical thermo-migration of confined air bubbles: in the presence of surfactant the bubbles can go the other way.   

Two-Dimensional Microfluidics: Stable Island Emulsions in Freely Suspended Smectic Liquid Crystal Films

Islands (circular regions of greater thickness) in smectic films are easily created and manipulated, but are generally unstable, tending to grow or shrink over time. We have recently created stable emulsions of smectic islands by ``work hardening'' of the smectic film using shear and extensional flow to form a dense, mechanically stable network of edge dislocations. In this talk, we discuss this novel type of two-dimensional colloidal system, in particular the island size distribution, network of edge dislocations and topological defects that form stable two-dimensional emulsions.   

Spherical and Non-Spherical Double Emulsions with Multiple Components

Monodispersed double emulsions, drops inside of drops, with multiple and tunable components are generated using microfluidics. A fluid dynamic model based on fast camera images of the single step emulsification technique is being developed to determine the critical separation between channels in the injection capillary; this model addresses the maximium number of distinct drops that can be controllably loaded inside a double emulsion for a given diameter capillary. New stable, non-spherical emulsions with two and three different components, Janus and Cerberus emulsions, are also reported.   

Stability of thin liquid films: Influence of interfacial viscoelasticity

Lipid layer spreading and liquid film dewetting are important variables influencing numerous processes, including the stability of the tear film. The viscoelasticity of insoluble monolayers may govern thin liquid film dewetting phenomena. The purpose of this work is to gain insight into the effects of surface viscoelasticity elasticity by insoluble monolayers on dewetting of thin films of water with a particular attention paid to materials, such as meibum, that stabilize the tear film. For this purpose an experiment has been devised wherein monolayers of known surface pressure and surface rheology are introduced atop thin, liquid films that would normally spontaneously dewet .The results reveal that monolayers of viscoelastic surfactants are able to stabilize thin films against spontaneous dewetting. As the surface pressure and surface rheology of these layers is increased, their effectiveness is enhanced. Meibum is particularly effective in stabilizing thin films. These results suggest that the role of the meibum is to offer the tear film enhanced stability and not only to suppress evaporation.   

Light-controlled air-water interfaces and thin liquid films using photo-surfactants

We study the interfacial behavior of photosurfactants containing an azobenzene moiety in the apolar tail --which switch from cis to trans conformation depending on the wavelength of light. We present here results on the effet of stimulation on the interfacial dynamics of such photosurfactants upon illumination. First, without light stimulus, the trans isomers is found to desorb more slowly than the cis, this leads to a fast enrichment of the interface with trans. Under light, adsorbed trans convert to their cis form which quickly desorb resulting in an important decrease of the surface excess and an increase of the surface tension. Besides, we stimulate thin-liquid films stabilized by such surfactants. Several types of hydrodynamical instabilities in the thin-liquid films are generated. We show that these instabilities are due to a strong rise of concentration in-situ but also a light-induced variation of DLVO interactions that usually stabilize the films.   

Supercritical Carbon Dioxide Assisted Processing of Silica/PMMA Nanocomposite Foams

Polymer nanocomposite foams receive considerable attention in both scientific and industrial communities. These structures are defined as closed or open cells (pores) surrounded by bulk material and are widely observed in nature in the form of bone structure, sponge, corals and natural cork. Inspired by these materials, polymer nanocomposite foams are widely used in advanced applications, such as bone scaffolds, food packaging and transportation materials due to their lightweight and enhanced mechanical, thermal, and electrical properties compared to bulk polymer foams. The presence of the nanosized fillers facilitates heterogeneous bubble nucleation as a result, the number of bubbles increases while the average bubble size decreases. Therefore, the foam morphology can be controlled by the size, concentration, and surface chemistry of the nanofiller. In the current study, we used supercritical carbon dioxide as a foaming agent for silica/poly(methyl methacrylate), PMMA, foams. The silica nanoparticles were chemically modified by fluoroalkane chains to make them CO$_{2}$-philic. The surface coverage was controlled via tethering density, and the effect of silica surface coverage and concentration on foam morphology was investigated through scanning electron microscopy and image processing. Results indicated that nanofiller concentration and filler surface chemistry (CO$_{2}$-philicity) had tremendous effect on foam morphology but surface coverage did not have any effect.   

Drainage dynamics of aqueous foams generated by sparging and turbulent mixing

We investigate the effect of bubble size on the drainage dynamics of aqueous fire-suppression foams using laboratory-scale foam generators and theoretical modeling. We generate foams over a wide range of bubble sizes using two foam generation methods--sparging using fritted sheets of steel, and turbulent mixing using high-pressure T-junctions. The sparged foams comprise bubbles of mean diameter 0.5 mm or larger and begin draining immediately whereas the turbulently mixed foams comprise bubbles of mean diameter 0.15 mm or smaller and begin draining after induction times of 5-15 minutes. We study two proprietary fire-suppression foam solutions: a non-fluorinated surfactant solution containing viscous additives intended for use as a wet foam, $i.e.$ liquid fraction $>$ 0.1, and a sodium dodecyl sulfate surfactant solution intended for use as a dry foam, $i.e.$ liquid fraction $<$ 0.005. The change in liquid retention time due to change in mean bubble size differs between these two solutions. We compare our experimental results with theoretical models to examine the reasons for the difference in liquid retention time.