We present a mean field theory to describe swelling behaviors of a nematic gel immersed in a low molecular weight liquid crystal solvent under an external field such as magnetic and electric fields. We calculate the equilibrium concentration of the gel, orientational order parameters, and elongation of the gel. As a result of a nematic ordering between the gel and solvent inside the gel, the shape of the gel is discontinuously (or continuously) elongated with increasing the strength of the external field. We examine the condition for a nematic ordering-induced first- and second-order volume phase transition of the gel.
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