The Shape of Inflated Vesicles

The conformation and scaling properties of self-avoiding fluid vesicles with zero extrinsic bending rigidity subject to an internal pressure increment $\mathrm{\Delta }p>0$ are studied using Monte Carlo methods and scaling arguments. With increasing pressure, there is a first-order transition from a collapsed branched polymer phase to an extended inflated phase. The scaling behavior of the radius of gyration, the asphericities, and several other quantities characterizing the average shape of a vesicle are studied in detail. In the inflated phase, continuously variable fractal shapes are found to be controlled by the scaling variable $x=\mathrm{\Delta }p{N}^{3\nu \mathrm{/}2}$ (or equivalently, $y = {<V>}/ N^{3\nu/2}$),where$N$isthenumberofmonomersinthevesicleand$V$ the enclosed volume. The scaling behavior in the inflated phase is described by a new exponent $\nu =0.787\pm 0.02$.