Free-Meniscus Coating Processes

Abstract

When faced with depositing a liquid film on a surface, laboratory scientists usually rely on free coating by withdrawal or drainage, or so-called free-meniscus coating. Simplistic and inexpensive, these techniques abound in the research and development of materials that can be deposited in the liquid state. What has come to be known as dip coating (Deryagin and Levi 1964; Ruschak 1976; Scriven 1988), viscous lifting (Van Rossum 1958), or drag-out (Landau and Levich 1942) begins with immersing a substrate in a vessel filled with liquid. Withdrawal of the substrate from the liquid, if managed properly, can result in a thin coherent liquid film, as shown in Fig. 13.1b. Alternatively, the liquid in the vessel can be drained around the substrate. This technique is often referred to as coating-by-drainage (Jeffreys 1930; Van Rossum 1958; Groenveld 1971) and is diagrammed in Fig. 13.1c. Recourse is sometimes taken from dip coating to coating-by-drainage when limited vertical space prevents substrate withdrawal (cf. Ashley and Reed 1984) or when coating small, short substrates to avoid the local thickening obtained when the trailing edge is withdrawn (Schroeder 1969). Although most often dip coating is operated as a batch process, it can be made continuous (Fig. 13.1d) when the substrate is a long, flexible sheet or filament (Deryagin and Levi 1964; Scriven 1988); however, sustaining a continuous coating process requires the addition of make-up liquid to the vessel. In any case, the physics of batch dip coating, continuous dip coating, and coating-by-drainage are essentially the same, differing only in the frame of reference in which the flat part of the meniscus is stationary.