Design of Mixed PDMS-mPEG Slippery Covalently Attached Liquid-Like Surfaces

Low droplet friction is desirable in many circumstances in which liquids interact with solid surfaces. This study explores the fabrication of surface-grafted, liquid-like layers with ultralow static droplet friction, made from a mixture of hydrophobic polydimethylsiloxane (PDMS) and hydrophilic methoxy polyethylene glycol (mPEG). These mixed layers are prepared via a two-step spin coating process in which reactive ethanol solutions are applied to the surface in sequence. Both polymers are liquid at room temperature and, when mixed, lead to slippery layers with contact angles that can be tuned from that of pure PDMS to that of pure mPEG. A contact angle hysteresis of 0.9 ± 0.3° was obtained on mPEG_9-12 layers. This is the lowest hysteresis reported for any hydrophilic covalently attached liquid surface and represents the lowest contact line friction ever observed on a solid planar surface. As the PDMS fraction in the mixed layer increased, so too did contact angle hysteresis, reaching a maximum value of 9° at 70% PDMS, before returning to 2° for the pure PDMS layer. Atomic force microscopy mapping of the liquid layers revealed that the two polymers are fully mixed on the surface, even at high surface fraction of both components. The model by Reyssat & Quéré, devised to explain contact angle hysteresis for surfaces with dilute defects, explains the observed results well. This study shows that liquid-like surfaces can be achieved that are more slippery than conventional self-assembled monolayers and share the same capacity to gradually tune surface wettability. These mixed layers are excellent model systems with which to study interfacial phenomena, such as wetting, adhesion, and friction, the interactions of proteins and cells with surfaces, and for applications, from increased heat transfer to efficient atmospheric water capture and antifouling.