Reversible switching of interfacial interactions

The key to assembling and manipulating materials ranging in size from molecules to microns involves controlling how the materials interact with each other or with patterned substrates. The focus of this review involves recent work in which researchers are learning how to use materials such as self-assembled monolayers (SAMS) to reversibly program interfacial interactions using external stimuli including heat, light, and electric fields. Using such stimuli, it has been shown that intermolecular and surface forces including electrical double layer interactions, hydration forces, π-stacking interactions, and hydrophobic/hydrophilic behavior can be switched back and forth between discrete states. Such switching allows surfaces to be programmed to grab or release generic classes of materials or specific objects based on programmed molecular recognition. This review highlights strategies for developing and characterizing responsive interfaces, shows how such surfaces have been exploited in microfluidic systems, and explores the promise of switchable materials for creating responsive and adaptable materials in three dimensions.