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.
Highlights
IntroductionThe interaction of droplets with engineered solid surfaces has relevance from both a fundamental and an applied perspective.On the one hand, understanding the mechanisms involved in the interaction between droplets and complex surfaces can unveil new physics in the context of solid−liquid interactions.On the other hand, engineered surfaces can be used to solve problems in applications such as ink-jet printing,[1] coating,[2] and lubrication.[3]Recently, there has been a sustained interest in slippery omniphobic covalently attached liquid-like (SOCAL) surfaces, which are a type of engineered, ultrasmooth solid surface that offers remarkably low static friction to the motion of droplets.[4−6] SOCAL surfaces are achieved by acid-catalyzed graft polycondensation of dimethyldimethoxysilane, where short polymer chains are covalently bound to a solid substrate creating a nanometric monolayer that shields a droplet from the underlying solid substrate.[4]
There has been a sustained interest in slippery omniphobic covalently attached liquid-like (SOCAL) surfaces, which are a type of engineered, ultrasmooth solid surface that offers remarkably low static friction to the motion of droplets.[4−6] SOCAL surfaces are achieved by acid-catalyzed graft polycondensation of dimethyldimethoxysilane, where short polymer chains are covalently bound to a solid substrate creating a nanometric monolayer that shields a droplet from the underlying solid substrate.[4]
During the injection phase, the apparent contact angle increases sharply from the initial value θi ≈ 103°. This sharp increase is followed by a steady motion of the contact line, where θ ≈ 106° and where the base radius grows at a rate ṙ = 9 ± 1 μm/s
Summary
The interaction of droplets with engineered solid surfaces has relevance from both a fundamental and an applied perspective.On the one hand, understanding the mechanisms involved in the interaction between droplets and complex surfaces can unveil new physics in the context of solid−liquid interactions.On the other hand, engineered surfaces can be used to solve problems in applications such as ink-jet printing,[1] coating,[2] and lubrication.[3]Recently, there has been a sustained interest in slippery omniphobic covalently attached liquid-like (SOCAL) surfaces, which are a type of engineered, ultrasmooth solid surface that offers remarkably low static friction to the motion of droplets.[4−6] SOCAL surfaces are achieved by acid-catalyzed graft polycondensation of dimethyldimethoxysilane, where short polymer chains are covalently bound to a solid substrate creating a nanometric monolayer that shields a droplet from the underlying solid substrate.[4]. The interaction of droplets with engineered solid surfaces has relevance from both a fundamental and an applied perspective. On SOCAL surfaces, droplets are subject to a very low contact-angle hysteresis, typically of 1° or below. Despite this low hysteresis, droplets on SOCAL surfaces exhibit a remarkably low mobility,[5] indicating an unexpected high dynamic friction imparted by the surface on a moving droplet. This raises important questions about the physical mechanism governing the motion of contact lines on SOCAL surfaces. The remarkable combination of low static friction but high dynamic friction can unlock applications in surface engineering, where SOCAL surfaces act as “low-pinning-slow shedding” coatings
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