Accueil > Séminaires > Précédents séminaires > Liquid slip and nano-scale free-interface flow on alkylsilane self-assembled monolayers

Liquid slip and nano-scale free-interface flow on alkylsilane self-assembled monolayers

Joshua D. McGraw Département de Physique, Ecole Normale Supérieure

Alkylsilane self-assembled monolayers (SAMs) are often used as model substrates for their ease of preparation and hydrophobic properties. These atomically smooth monolayers provide a slip boundary condition for dewetting films composed of unentangled polymers. This slip length, an indirect measure of the friction between a given liquid and different solids, is switchable if the alkyl chain length is changed from 12 to 18 backbone carbons. This change in boundary condition is typically affected in a quantized way, using one or the other alkyl chain length, thus obtaining one or the other slip length. Here, we present results in which this SAM structure is changed in a continuous way. We prepare SAMs containing bidisperse mixtures of alkyl silanes with the composition as a control parameter, and thus observe an evolution in the slip length over nearly two orders of magnitude on changing the SAM composition. With the ability to control slip using SAMs, we also open the door to understanding the effects of slip in the evolution of nano-scale liquid droplets. Using in-situ atomic force microscopy, we have captured the evolution towards equilibrium of the initially unstable droplets. Scaling and numerical analysis of the hydrodynamical governing laws are compared to the experimental data. We find good agreement by using the slip length as a governing parameter. At low slip we find that the data obeys classical arguments, with dissipation localized near the intersection between the liquid/vapour and solid/liquid interfaces. At large slip, however, we observe a transition to nearly frictionless droplet dewetting and dissipation spreads throughout the bulk of the droplet.