Penetration and adsorption of a water droplet causing local deformation of the poly(methyl methacrylate) surface.

A molecular dynamics approach has been employed to study the wetting behavior of poly(methyl methacrylate) (PMMA). The flexible and rigid models of PMMA substrate are compared. The results verify that the rigid model is not suitable to act as the PMMA substrate in simulation because it prevents the diffusion of PMMA molecules, which affects the penetration behavior of water molecules. The contact angle of different sizes of water droplets on the PMMA surface and local root-mean-square roughness have been calculated. The local root-mean-square roughness increases with decreasing size of water droplet. As the number of water molecules in the droplet falls below 1000, the contact angle of the water droplet and the permeability of the PMMA substrate are affected significantly because of the lower adsorption energy between the water molecules and the diffusion of the outmost PMMA molecules on the surface. When the droplet is composed of more than 1000 water molecules, the contact angle shows an agreement with experimental results. Finally, the hydrogen-bonded number as a function of distance along the Z direction and the distribution of the hydrogen-bonded number and adjacent number in different regions are discussed, with results showing that the hydrogen-bonded number is in inverse proportion to the number of water molecules of the droplet in the region below the dividing surface.