Dynamic wetting phenomena and contact angle hysteresis on polymer brushes and gels

Droplet mobility on surfaces is obtained by polymeric coatings, like gels or brushes. Both types hold some amount of un-crosslinked or un-grafted polymer (‘free’) chains which are mobile within the coating. In the presence of droplets, these free chains accumulate around the three-phase contact line, leading to contact lubrication and wetting ridge formation. When droplets move over the surface, the wetting ridge moves accordingly, yielding visco- and poroelastic responses of the surface. The coupling between surface response, free chain reorganization, and the sliding droplet is key to understanding liquid repellency. However, investigations are hampered by the large-scale separation of free chains (nanometer), surface (micrometer), and droplet (millimeter). Here, we utilize confocal laser scanning microscopy and interferometry to directly visualize the wetting ridge during droplet motion. Those methods let us distinguish between phases and enable optical resolutions below a micron. A novel optical force sensor let us measure the evolving friction forces between droplets and surface which occur in the order of micro Newton. We show that friction forces scale with velocity and contact lubrication. The presented techniques give new methods to directly determine the coating rheology of soft surfaces.