Spreading or contraction of viscous drops between plates: single, multiple or annular drops

The behaviour of a viscous drop squeezed between two horizontal planes (a Hele-Shaw cell) is treated by both theory and experiment. When the squeezing force $F$ constant surface tension neglected, predicts ultimate growth radius $a\sim t^{1/8}$ with time $t$ . This first reviewed found to be in excellent agreement Surface at boundary reduces interior pressure, this effect included analysis, although it negligibly small experiments. An initially elliptic tends become circular as increases. More generally, evolution stable under perturbations. If, on other hand, reversed ( $F<0$ ), so that plates are drawn apart (the ‘contraction’, or ‘lifting plate’, problem), subject fingering instability scale determined tension. trapped air bubble centre then considered. annular still follow ‘one-eighth’ power law, but unstable, originating bubble, i.e. inner annulus. realised experimentally ways: simply starting form an annulus, nearly possible; second forcing four separate drops expand merge, process involves resolution ‘contact singularities’ If apart, driven from outer levering one corner: develops spreads rapidly slowly increased. At later stage, before rupture film complete separation plates, also any cavitation bubbles appear very low-pressure region, far point leverage. exotic discussed light foregoing theoretical analysis.