The widespread application of ionic hydrogels in a number of applications like control of microfluidic flow, development of muscle-like actuators, filtration/separation and drug delivery makes it important to properly understand these materials. Understanding hydrogel properties is also important from the standpoint of their similarity to many biological tissues. Typically, gel size is sensitive to outer solution pH and salt concentration. In this paper, we develop models to predict the swelling/deswelling of hydrogels in buffered pH solutions. An equilibrium model has been developed to predict the degree of swelling of the hydrogel at a given pH and salt concentration in the solution. A kinetic model has been developed to predict the rate of swelling of the hydrogel when the solution pH is changed. Experiments are performed to characterize the mechanical properties of the hydrogel in different pH solutions. The degree of swelling as well as the rate of swelling of the hydrogel are also studied through experiments. The simulations are compared with experimental results and the models are found to predict the swelling/deswelling processes accurately. The validated models are then used to study the influence of the various hydrogel and solution parameters on the degree and rate of swelling/deswelling of the hydrogel. [785]

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In this Letter, the formation of complex oscillations of the type 2n M oscillations per period at the Mth superharmonic excitation is reported for electrostatic microelectromechanical systems. A dc bias (beyond "dc symmetry breaking") and an ac signal (at the Mth superharmonic frequency) with an amplitude around "ac symmetry breaking" gives rise to M oscillations per period or period M response. On increasing the ac voltage, a cascade of period doubling bifurcations take place giving rise to 2n M oscillations per period. An interesting chaotic transition (1-band and 2-band chaos) is observed during the first period doubling bifurcation. The nonlinear nature of the electrostatic force is shown to be responsible for the reported observations.