Interfacial aspects of the stability of polyglycerol ester covered bubbles against coalescence

Abstract

Most liquid foams are not stable, with one destabilization process being coalescence. The current study aims at better understanding the coalescence of bubbles covered by the non-ionic surfactant polyglycerol ester (PGE) in dilute solutions. Large deformations of bubble surfaces are of importance in understanding coalescence as they can induce coalescence events. Furthermore, during coalescence and the subsequent shape relaxation large deformations are imposed on the surfaces. These large deformation experiments were carried out using interfacial shear and dilatational rheology. Furthermore isolated coalescence events of bubbles were observed. The largest fraction of the interfacial shear modulus was recovered within 1 minute of halting the large deformations, with the overall recovery happening on a timescale of hours. When mimicking coalescence by reducing the surface area, a short-term increase in the elastic modulus was observed in dilatational rheology. The remarkably slow, two-mode exponential shape relaxation recorded for sufficiently covered bubbles following their coalescence hints at important interfacial structuring phenomena. As a result of the low solubility of PGE, coalescence appears to help in achieving a certain stability level at short timescales via an increase in surface coverage and the growth of multi-layer domains. Finally, coalescence patterns in two-dimensional foams were investigated, which suggested that in the absence of prior coalescence, very long adsorption times (>12 hours) are required to reach significant stability. These experiments also revealed a remarkable stability of polyglycerol ester foams against Ostwald ripening.