Temperature and time dependencies of surface patterns in constrained ionic N-isopropylacrylamide gels

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Abstract
Surface patterns in ionic N-isopropylacrylamide (N-IPA) gels have been investigated under external constraint. Hexagonal, grain, and bubble patterns have been observed at temperatures below, near, and above the phase transition temperature Tc. It is found that the behavior of these patterns depends not only on temperature, time, and external constraint, but also on the thermal path. The time dependence of the gel thickness has revealed that there is a plateau period and that the end of the plateau period corresponds to the onset of the bubble pattern. This observation is in agreement with previous measurements of free spherical N-IPA beads. The experiments show that each hexagonal cell evolves into a bubble for N-IPA gels, while a bubble can be present in the middle of a hexagonal cell for acrylamide gels. The mechanism for the shrinking patterns will be also discussed.

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