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Gradient of osmotic pressure generally induces a motion in solutions and gels.For example, if two solutions having different concentrations are separated by a semi-permeable which allows solvent to pass through but prohibits solute to pass through, a flow is induced in the solution which tends to equilibrate the chemical potential of solvent.Similarly if a salty solution is placed on top of a sheet of a hydrogel, the gel sheet bends temporarily, and then bends back.Here I discuss such motion using the Onsager principle, and demonstrate the utility of the principle in soft matter dynamics.