Contact-Angle Hysteresis and Contact-Line Friction on Slippery Liquid-like Surfaces

Abstract

Contact-line pinning and dynamic friction are fundamental forces that oppose the motion of droplets on solid surfaces. Everyday experience suggests that if a solid surface offers low contact-line pinning, it will also impart a relatively low dynamic friction to a moving droplet. Examples of such surfaces are superhydrophobic, slippery porous liquid-infused, and lubricant-impregnated surfaces. Here, however, we show that slippery omniphobic covalently attached liquid-like (SOCAL) surfaces have a remarkable combination of contact-angle hysteresis and contact-line friction properties, which lead to very low droplet pinning but high dynamic friction against the motion of droplets. We present experiments of the response of water droplets to changes in volume at controlled temperature and humidity conditions, which we separately compare to the predictions of a hydrodynamic model and a contact-line model based on molecular kinetic theory. Our results show that SOCAL surfaces offer very low contact-angle hysteresis, between 1 and 3°, but an unexpectedly high dynamic friction controlled by the contact line, where the typical relaxation time scale is on the order of seconds, 4 orders of magnitude larger than the prediction of the classical hydrodynamic model. Our results highlight the remarkable wettability of SOCAL surfaces and their potential application as low-pinning, slow droplet shedding surfaces.

Surface morphology effects on clathrate hydrate wettability

“…We hypothesize that surface heterogeneities, such as roughness (at various length scales), could affect the measured contact angle, particularly noting that the hydrate surfaces used by Thomas et al were very smooth, rigid, and flat [25], and noting that Young's equation (see Figs. 2 and 3), is only applicable to smooth and flat surfaces [23,50]. Our hypothesis is supported by the experimental observations by Stoner et al [28], and also by Spencer et al [77], who quantified the effect of heterogeneity on contact line pinning.…”

“…which relates the contact angle to the surface free energies of solid-liquid (c sf2 and c sf1 ) and liquid-liquid (c f1,2 ) interfaces [23,50]. Simulated Interfacial Tensions.…”

Lossless, Passive Transportation of Low Surface Tension Liquids Induced by Patterned Omniphobic Liquidlike Polymer Brushes

“…Liquidlike polymer brushes have demonstrated extremely low CAH toward various LST liquids. [22][23][24] We designed two liquidlike brushes, one based on polydimethylsiloxane (PDMS) and the other on perfluoropolyether (PFPE), for our channel and background surface chemistries, respectively. The low contact line pinning of droplets on these coatings is demonstrated in Figure S1 of the Supporting Information, where droplets of toluene easily slid when the surfaces were tilted only a few degrees (1.5° for PDMS and 2.5° for PFPE).…”

Control of droplet evaporation on smooth chemical patterns

“…This limit is particularly relevant for applications as this type of surfaces are easier to implement experimentally. While chemical patterning typically leads to pinning points on a solid surface, recent progress has been made in creating liquid-like surfaces [18] and liquid-infused surfaces [19] with very low pinning. Such surfaces could be used to study a continuous variation of the surface wettability as an experimental system to explore the ideas presented in this paper.…”