Molecular Gradients on Surfaces: Formation and Applications in Soft Condensed Matter Science

Recent advances in the field of self-assembly have led to the development of a plethora of new technologies based on soft lithography (see Self-assembled Monolayer Films: Microcontact Printing; Thiol-based Selfassembled Monolayers: Formation and Organization; Thiol-based Self-assembled monolayers, Structure of) that enable alternative ways of creating twoand three-dimensional chemical patterns on material surfaces. Many soft lithography techniques are based on controlled deposition of self-assembled monolayers (SAMs) (see Self-assembled Monolayers by Evaporation). Various structural patterns with dimensions ranging from hundreds of nanometers to several micrometers are created on the material surface using a ‘‘pattern-transfer element’’ or stamp that has a three-dimensional structure molded onto its surface. While useful for decorating substrates with welldefined chemical patterns of various shapes and dimensions, the soft-lithography technologies always produce sharp boundaries between the distinct chemical regions on the substrate. For some applications, it is desirable that the physicochemical characteristics, such as wetting of the substrate, change gradually. This can be accomplished by producing surfaces with a position dependent and gradually varying chemistry. In these so-called ‘‘gradient surfaces,’’ the gradient in surface energy is responsible for a position-bound variation in physical properties, most notably the wettability. These gradient substrates can be useful in highthroughput studies of the interfacial behavior of molecules and macromolecules (the entire behavioral spectrum can be accessed in a single experiment), can serve as templates for further processing, or be used as active elements in controlled surface transport of materials. Methodologies that have been developed since the 1960s to produce such gradient substrates will be discussed and several recent applications of such gradient substrates in soft condensed matter field will be presented.