Hydrophilic and superhydrophilic surfaces and materials

Abstract

The term superhydrophilicity is only 11-12 years old and was introduced just after the explosion of research on superhydrophobic surfaces, in response to the demand for surfaces and coatings with exceptionally strong affinity to water. The definition of superhydrophilic substrates has not been clarified yet, and unrestricted use of this term to hydrophilic surfaces has stirred controversy in the last few years in the surface chemistry community. In this review, we take a close look into major definitions of hydrophilic surfaces used in the past, before we review the physics behind the superhydrophilic phenomenon and make recommendation on defining superhydrophilic surfaces and coatings. We also review chemical and physical methods used in the fabrication of substrates on surfaces of which water spreads completely. Several applications of superhydrophilic surfaces, including examples from the authors' own research, conclude this review.

Physics and applications of superhydrophobic and superhydrophilic surfaces and coatings

“…3 However, it is known that rain droplets do not spread to form a water film on the windows and windshields, but spread only partially. The contact angle is typically 10-30° -as a result of surface contamination with dirt and air-borne organics ( Figure 2).…”

“…3 For example, Gao and McCarthy 49 doubted the validity of both the Wenzel and Cassie-Baxter approaches and argued that contact line, and not the contact area, is important in interpretation of the contact angles. A similar conclusion was also drawn earlier for chemically heterogeneous surfaces.…”

“…Such surfaces have at least atomic heterogeneity that can trigger adsorption of constituents from liquid or gas adding to heterogeneity of the surface and its landscape. 3 Some of the solids can also react chemically 20,21 or interact physically with the probing liquid or surrounding gas and promote surface segregation, reorientation, diffusion or migration. 22,23 Dissolution of pharmaceutics, soluble and semisoluble minerals and many inorganic and organic reagents/products makes the wetting characterization of such materials extremely difficult and sometimes impossible.…”

“…22,23 Dissolution of pharmaceutics, soluble and semisoluble minerals and many inorganic and organic reagents/products makes the wetting characterization of such materials extremely difficult and sometimes impossible. 3 In addition, as spreading is a timedependent event, the conditions between solid and spreading liquid can vary depending on the time of liquid shape observation during contact angle measurements. Softer materials also deform under the weight of the probing liquid.…”

“…[24][25][26][27][28] This results in an increase in the force required to slide a drop on the solid surface as can be measured using the centrifugal adhesion balance. 3,4,26,29 Mutual saturation between phases or preferential adsorption at interfaces can be very fast, but sometimes, it occurs as a very slow process and thus exceeds the time of contact angle measurements, resulting in data measured under non-equilibrium conditions. 30 A good protocol requires contact angle measurements in liquidsaturated gas environment to at least secure equilibrium solid surface conditions.…”