Digital holographic microscopy for measurement of instantaneous contact angle of an evaporating droplet

Despite the apparent simplicity of the process of a droplet evaporating on a substrate, the temporal evolution of the local contact angle along the droplet periphery during the evaporation is still not fully understood. In this work, digital holographic microscopy is used for capturing the time evolution of the droplet surface at submicron resolution. The three-dimensional surface provides the information for determining the instantaneous three-phase contact line and the local apparent contact angle along the whole droplet periphery. The apparent contact angle is computed from the gradient of the droplet surface and defined as the maximum slope of this function along the periphery of the droplet. The method is then applied to study the diffusion dominated evaporation of nanoliter droplets on a hydrophilic substrate. For the case of a non-axisymmetric droplet, differences of up to 10° in contact angle along the contact line were identified when the contact line went from one pinning position to another. Digital holographic microscopy allowed us to characterize the irregular motion of the contact line and local changes in contact angle during the evaporation process.