Drying Dip-Coated Colloidal Films

Abstract

We present the results from a Small Angle X-ray Scattering (SAXS) study of lateral drying in thin films. The films, initially 10 µm thick, are cast by dip-coating a mica sheet in an aqueous silica dispersion (particle radius 8 nm, volume fraction φs = 0.14). During evaporation, a drying front sweeps across the film. An X-ray beam is focused on a selected spot of the film, and SAXS patterns are recorded at regular time intervals. As the film evaporates, SAXS spectra measure the ordering of particles, their volume fraction, the film thickness and the water content, and a video camera images the solid regions of the film, recognized through their scattering of light. We find that the colloidal dispersion is first concentrated to φs = 0.3, where the silica particles form a gel with repulsive interactions. Then the particles aggregate until they form a cohesive wet solid at φs = 0.68 ± 0.02. Further evaporation from the wet solid leads to evacuation of water from pores of the film, but leaves a residual water fraction φw = 0.16. The whole drying process is completed within three minutes. An important finding is that, in any spot (away from boundaries), the number of particles is conserved throughout this drying process, leading to the formation of a homogeneous deposit. This implies that no flow of particles occurs in our films during drying, a behavior distinct to that encountered in the iconic coffee-stain drying. It is argued that this type of evolution is associated with the formation of a transition region that propagates ahead of the drying front. In this region the gradient of osmotic pressure balances the drag force exerted on the particles by capillary flow toward the liquid-solid front. INTRODUCTIONCoatings are often made through deposition of liquid colloidal dispersions. Common examples are paints, anticorrosion coatings and ceramic coatings. In most cases, the dispersion is applied as a liquid film, and it changes into a solid film as a result of solvent evaporation. A variety of deposition patterns can be obtained, depending on the evaporation profile over the liquid film and on the flows taking place inside the deposit during drying [1][2][3][4][5][6][7]. The two extreme situations are as follows:(a) Homogeneous film formation: in the simplest case, there is no lateral flow through the liquid film, and all volatile components are evaporated locally. The number of particles at each point of the film is conserved throughout the drying process. Therefore the deposition of a uniform liquid colloidal film leads to a final solid film that has homogeneous composition, thickness and microstructure. Relatively homogeneous films can also be obtained through mechanisms where there is a moving liquid-solid boundary, and there is a flow of liquid phase near this boundary [8,9].(b) Coffee-ring: this process has also been extensively studied [1, 2]. In this situation a macroscopic flow throughout the liquid carries nearly all the particles to the fixed edges of the film, where they pack into a dense rim. At the end o...

Drying colloidal systems: Laboratory models for a wide range of applications

“…Evaporation on the large surface of the latter zone induces a significant lateral flow from the other side of the front. This flow brings colloids to the dense zone, allowing its growth and therefore the front shift [7]. In this front region, the dense layer assembly is flow-driven in a manner similar to what is observed in filtration.…”

Structural anisotropy of directionally dried colloids

“…We used Levasil-30 (Azko-Nobel, particle radius a = 46 nm) as received, and dialysed Ludox SM-30 (a = 5 nm), HS-40 (a = 8 nm), and TM-50 (a = 13 nm) against aqueous solutions of NaCl and NaOH (pH 9.5), as in Ref. [8]. Poly(ethylene glycol) was added to the dialysis buffer, to concentrate the dispersions by osmotic stress.…”

Plasticity and Fracture in Drying Colloidal Films

“…(1) and (2). For covered films, evaporation rates of order 10 À9 m=s are reported [25], an order of magnitude lower than over uncovered films [26]. A crack could be left covered for 15 min without noticeable growth, whereas for uncovered films a crack tip typically grew out of the microscope field of view within a minute.…”