Session R4: Foams

Surface waves in a foam

We investigate the propagation and attenuation of waves generated at the surface of a liquid foam after the impact of a solid sphere. Surface deformation is recorded with high spatial and temporal resolution thanks to a fringe projection technique. We show that most surface waves travel at a velocity of a few meters per second. High velocity impacts also trigger the emission of faster waves. We discuss the nature of these two types of waves.   

The effect of gravity on drainage and rupture in surfactant-free foams

In low liquid-fraction surfactant-free foams, lamella thinning due to drainage leads to rupture by van der Waals instability, which causes coarsening due to coalesence of neighboring bubbles. We use asymptotic analysis to predict the effect of gravity on drainage flow for a flat, vertical lamella and for a weakly bent, horizontal lamella. In both cases the films thin non-uniformly, and the thinning is exponential for long times; much faster than the power law thinning rate predicted for gravity-free lamella drainage. The asymptotic solutions are also able to predict the onset of rupture in both geometries. Numerical solutions are used to verify these asymptotic predictions, and to determine their range of validity for relevant parameters.   

Coalescence driven coarsening in surfactant-free foams

We consider the stability of a planar gas-liquid foam with low liquid fraction, in the absence of surfactants and stabilising particles, as a model for molten metal foams produced as a precursor to forming high-porosity metallic solids. We adopt a network modelling approach, treating the bubbles as polygons, the accumulation of liquid at the bubble vertices as dynamic nodes and the liquid bridges separating the bubbles as uniformly thinning free films. We further incorporate an explicit rupture criterion for the films once they become sufficiently thin, due to van-der-Waals intermolecular attractions. We initialise the foam as a mono-disperse array of regular hexagonal bubbles, and examine the rate of coarsening as the films break and the bubbles rapidly coalesce.   

Particle-tracking velocimetry analysis of liquid drainage within individual Plateau borders in aqueous foam

Foam drainage theory describes macro-scale liquid drainage for a body of foam based on the microscopic flow within individual Plateau borders and within the nodes which occur at the intersections of multiple Plateau borders. The present study measures micro-scale liquid velocities within individual Plateau borders using microparticle image velocimetry, and it measures macro-scale liquid drainage using a weighing scale. Measurements take place over the course of free drainage for foam which is initially wet with initial liquid fraction equal to 20{\%} averaged over the height of the foam. Preliminary results show that the flow dynamics within individual Plateau borders evolve according to similar trends as the macro-scale volume of liquid drained. Foam drainage theory agrees with both measurements after an initial transition period, but during initial drainage the experimentally measured drainage rate exceeds that predicted by the theory. We discuss implications of the agreement between the micro-scale and macro-scale measurements as well as potential sources for the unexpectedly high drainage rate which occurs at the beginning of drainage.   

Self healing: solid spheres impacting soap bubbles

Under the right conditions a moving sphere may pass through a stationary soap bubble without rupturing it. At impact, the sphere forms a cavity in the soap film that often facilitates reparation after collapse. This interaction leaves a small film surrounding the sphere as it passes through the center of the bubble. In contrast, as the sphere passes through the opposite side of the bubble, rupture is more likely. The physics behind this phenomenon are not well understood, nor the limiting factors of this interaction. We explore the phenomenon using high-speed photography. Our observations reveal that there are several distinct cavity regimes. We present the parameters for drainage, rupture and reparation each of which are related to curvature gradients.   

Numerical Modeling of Nanocellular Foams Using Classical Nucleation Theory and Influence Volume Approach

Nanocellular porous materials present unusual optical, dielectric, thermal and mechanical properties and are thus envisioned to find use in a variety of applications. Thermoplastic polymeric foams show considerable promise in achieving these properties. However, there are still considerable challenges in achieving nanocellular foams with densities as low as conventional foams. Lack of in-depth understanding of the effect of process parameters and physical properties on the foaming process is a major obstacle. A numerical model has been developed to simulate the simultaneous nucleation and bubble growth during depressurization of thermoplastic polymers saturated with supercritical blowing agents. The model is based on the popular ``Influence Volume Approach,'' which assumes a growing boundary layer with depleted blowing agent surrounds each bubble. Classical nucleation theory is used to predict the rate of nucleation of bubbles. By solving the mass balance, momentum balance and species conservation equations for each bubble, the model is capable of predicting average bubble size, bubble size distribution and bulk porosity. The model is modified to include mechanisms for Joule-Thompson cooling during depressurization and secondary foaming. Simulation results for polymer with and without nucleating agents will be discussed and compared with experimental data.   

A novel method of producing stable emulsions via electrified W/O interfaces

In the current paper a vertical electric field was induced to the liquid interface to make thin jets from the conical tip structures. By the means of this jet dispersion a novel method of emulsification of water drops in dielectric medium was represented. Experiments reported in this paper enable a comprehensive illustration of introduced mechanism for the emulsification. The important aspects of an emulsion production were investigated through produced droplets properties, including their movement velocity, and size distribution. Variation of mentioned parameters was investigated with the conductivity throughout various electrolyte ion concentration additions to the dispersed phase. Experiments show that, conductivity augmentation antedates the stable cone-jet formation to the lower electric fields. Furthermore, it reduces the stability duration of emulsified drops due to increasing in polydispersity and coalescence rate.