The behaviour of a viscous drop squeezed between two horizontal planes (a squeezed Hele-Shaw cell) is treated by both theory and experiment. When the squeezing force $F$ is constant and surface tension is neglected, the theory predicts ultimate growth of the radius $a\sim t^{1/8}$ with time $t$. This theory is first reviewed and found to be in excellent agreement with experiment. Surface tension at the drop boundary reduces the interior pressure, and this effect is included in the analysis, although it is negligibly small in the squeezing experiments. An initially elliptic drop tends to become circular as $t$ increases. More generally, the circular evolution is found to be stable under small perturbations. If, on the other hand, the force is reversed ($F<0$), so that the plates are drawn apart (the ‘contraction’, or ‘lifting plate’, problem), the boundary of the drop is subject to a fingering instability on a scale determined by surface tension. The effect of a trapped air bubble at the centre of the drop is then considered. The annular evolution of the drop under constant squeezing is still found to follow a ‘one-eighth’ power law, but this is unstable, the instability originating at the boundary of the air bubble, i.e. the inner boundary of the annulus. The air bubble is realised experimentally in two ways: first by simply starting with the drop in the form of an annulus, as nearly circular as possible; and second by forcing four initially separate drops to expand and merge, a process that involves the resolution of ‘contact singularities’ by surface tension. If the plates are drawn apart, the evolution is still subject to the fingering instability driven from the outer boundary of the annulus. This instability is realised experimentally by levering the plates apart at one corner: fingering develops at the outer boundary and spreads rapidly to the interior as the levering is slowly increased. At a later stage, before ultimate rupture of the film and complete separation of the plates, fingering spreads also from the boundary of any interior trapped air bubble, and small cavitation bubbles appear in the very low-pressure region, far from the point of leverage. This exotic behaviour is discussed in the light of the foregoing theoretical analysis.
A study has been made of a number of epoxides of potential and current utility for the treatment of cotton to produce crease recovery and wash-and-wear properties. The prop erties imparted to cotton by these products vary greatly with respect to the color of the fabric, the retention of strength for a given level of crease recovery, the rate of hydrolysis of the epoxide in the treating bath, etc. It is in fact unreasonable to consider all epoxides as a class. since they differ as much from one another as they do from the conventional nitrogen-containing finishes. In general, however, they demonstrate resistance to chlorine retention and a high level of durability to all types of laundering, including that used by commercial launclries. One of the most promising epoxides examined was vinylcyclohexene dioxide, which imparted a desirable combination of properties to cotton when used alone or in 2: 1 blends with most commercially used nitrogenous resins. Some treatments of cotton were performed with 1,1,3-tris-(2,3-epoxypropoxy)-hutane, This tri-epoxide contains an acid-labile acetal linkage, which can be readily hydrolyzed. The properties of cotton treated with this epoxide were examined before and after hy drolysis. and provided confirmatory evidence for the important role of cross-linking in improving the properties of cellulose with resin treatments.
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