Free energy landscape of Janus nanoparticles at a liquid/vapor interface

While homogeneous colloidal particles select their position at an interface based on density and equilibrium contact angle alone, surface heterogeneities can produce multiple local equilibria with varying immersion depths and particle orientation, and hysteretic behavior in dynamic situations. The understanding and potential control of colloidal configurations is facilitated by the free energy landscape – the particle’s free energy as a function of its immersion depth and orientation – which is easily calculated in the continuum approximation assuming a flat liquid/vapor interface. Using molecular dynamics simulations for a Janus nanoparticle, we calculate the molecular-level free energy landscape by thermodynamic integration, incorporating both thermal fluctuations and bending of the interface due to local wetting angles on the particle surface. We find significant differences between continuum and molecular free energies surfaces both in numerical values and in the presence of local minima, and illustrate the distinction by simulations of particle migration to the interface from either the vapor or the liquid phase.