Wettability Gradients on Soft Surfaces

Properties, behaviors, and applications of soft materials depend decisively on the characteristics of their surfaces. Physical features and chemical functionality of the soft surfaces control their interactions with the surroundings thereby deciding their responses to various physical and chemical phenomena. A gradient of such surface features i.e, a gradual change in a chemical or physical characteristic across a surface will result in a gradual change in the response of the surface to its surroundings in the same direction. Chemical as well as physical (morphological) gradients on soft surface enable useful properties pertinent to a variety of fields such as microfluidics, surface coatings, sensing, optics, and biology. Numerous methods have been used for the preparation of chemical as well as morphological gradients. Practical applications of soft surface gradients require stable large-scale surfaces with precisely controlled directionality and resolution of the gradients. Wettability gradients are one of the prominent classes of gradients created on soft surfaces. These gradients are constituted by gradual increase or decrease of hydrophobicity/hydrophilicity across a surface. One-dimensional (1D) as well as two-dimensional (2D) wettability gradients are fabricated with different patterns. This short review will summarize the advancements in the preparation, properties, and applications of wettability gradients on soft surfaces. Qualitative description of the fabrication processes, properties, and practical applications of the gradients are included along with our comments about the future prospects of these systems.