DIFFUSION AND EVAPORATION CONTROL THE SPREADING OF VOLATILE DROPLETS ONTO SOLUBLE FILMS

Abstract

The spreading dynamics of a water droplet on a soluble polymer substrate is controlled by the rate of water transfers into the substrate. We provide a comprehensive description of the parameters controlling the wetting dynamics based on the analysis of the transfers between the spreading liquid and the solid substrate. Our model is supported by experimental results obtained on supported films of polymer with thickness e varied over two decades, on which droplets of water spread with a velocity U spanning two decades. Three different transfers are governing the hydration dynamics: (i) Water evaporates from the droplet, condenses on the substrate, and further diffuses through the polymer. (ii) Liquid water at the contact line diffuses in the polymer ahead of the contact line. (iii) Water in the droplet is convected at the droplet velocity U . The evaporative process is the most efficient at hydrating the substrate and controls the hydration of the polymer at distances ranging between a macroscopic cutoff length L and a microscopic length κ. L is set by the diffusion of vapor in air. κ results from a balance between the diffusion of water from the droplet at the contact line and the diffusion of water in the atmosphere. By analyzing the hydration profile at all distances to the contact line between L and κ, we define three different spreading regimes: a thick film regime where the film behaves as a semi-infinite medium, a thin film regime where hydration is homogeneous along the film thickness, and an intermediate regime where both situations coexist along the film away from the contact line.