Transient surface patterns during adhesion and coalescence of thin liquid films

Surface deformations during the coalescence of two polymer melt films were studied by use of a surface forces apparatus (SFA). Well-ordered periodic surface ripple/finger patterns were observed during the adhesion and coalescence, which eventually disappeared, leaving smooth polymer-air interfaces. The life-times of these transient well-ordered patterns depend on the viscosity and film thickness of the polymer melts. These observations are in contrast to the conventional understanding that liquid-liquid coalescence usually occurs with the deforming surfaces remaining smoothly curved at all stages, with no esoteric shape-transitions. The results reveal a new feature associated with liquid-liquid adhesion/coalescence, which may be of key importance for a full understanding of coalescence processes. We propose an explanation for the observed phenomenon in terms of simple physical concepts, and discuss other microscopic and macroscopic (including biological) systems where similar effects are likely to occur.