Preparation of Sol−Gel Films by Dip-Coating in Extreme Conditions

Abstract

Dip-coating of sol-gel solutions is a complex dynamic process that is difficult to model because it is associated with time-dependent evaporation-induced concentration and viscosity gradients in the solution. It is, however, highly used in the coating technology because it is simple and provides excellent reproducibility. Existing fair models have been proposed some decades ago to describe this method, but they are based on Newtonian and nonevaporating liquids and require several important assumptions and simplifications. In this work, we present a simple experimental study of sol-gel film formation by dip-coating, through which we propose a general semiexperimental model to predict the final film thickness. Spectroscopic ellipsometry was used as the main technique to obtain the film physical thickness and optical density for various dip-coating processing conditions (withdrawal speeds from 0.01 to 20 mm · s -1 and temperatures from 25 to 60°C) and for several different chemical solutions (TiCl 4 , TEOS, and MTEOS, all in the presence, or not, of block PEO-b-PPO copolymer templates in EtOH/H 2 O, with concentrations from 10 -1 to 10 -3 mol · L -1 ). We show that phenomena that are difficult to assess during deposition, such as viscosity variation, evaporation cooling, chemical reaction, and thermal Marangoni flow, may not have to be taken into account. The influences of various experimental parameters are discussed together with the limitations and the full potentiality of the dip-coating technique. We show that two regimes of film formation independently exist at extreme withdrawal speeds, while they combine into a third regime at intermediate speeds. Although the first regime is well-known and is governed by gravity-induced viscous drag at higher speeds, the second one is barely used and is governed by interdependent evaporation and capillarity rise at lower speeds. We show that both regimes can be selected to build up films with a tunable thickness and that a minimum thickness exists for each given solution at a critical speed for which we believe that the capillarity rise effect perfectly counterbalances the viscous drag. We also show that the capillarity regime is well-suited when one needs to deposit thick films from highly diluted solutions.

Novel synthesis of thick wall coatings of titania supported Bi poisoned Pd catalysts and application in selective hydrogenation of acetylene alcohols in capillary microreactors

“…24 Depending on the withdrawing speed, three deposition regimes can be observed: the capillary, the draining or a combination of both regimes. 25 The capillary regime is observed for very slow withdrawing speeds (<0.1 mm/s) and is associated with the capillary rise in the formed solid film. The draining regime, on the contrary, dominates at high withdrawing speeds (> 1mm/s) due to quick dragging of the viscous fluid.…”

Evaporation‐Directed Crack‐Patterning of Metal–Organic Framework Colloidal Films and Their Application as Photonic Sensors

“…This correlation is reported in ) in the so-called "capillary regime". [13] The periodicity of the parallel crack can be tuned from 47 down to 2μm by increasing the withdrawal speed from 0.02 to 1 mms -1 respectively (at room temperature). According with the literature [7][8] the crack periodicity is proportional to the film's thickness; this correlation was verified here, with a periodicity/thickness ratio equal to 3.5 for deposition on silicon wafers ( Figure S6).…”

Surface-induced assembly of sophorolipids

“…Previous studies by Faustini et al 71 have shown that experimental conditions are critical during dip-coating, and in particular the withdrawal speed (ws): at very low and very high ws, 0.01 mm s À1 and 10 mm s À1 respectively, the thickness of the film was supposed to be the highest, while a speed in the range 0.1-0.5 mm s À1 should help to achieve the thinnest film deposition. Initial experiments were performed at 25 1C with a relative humidity of 25% and a ws of 1 mm s À1 .…”

“…At a very low speed (0.01 mm s À1 ), the film formation was ruled by the capillary regime wherein solvent evaporation was faster than the motion of the drying line. 71 The film was then thick and the molecules could adopt less energetic organization on the surface. On the contrary, at high withdrawal speed (10 mm s À1 ), the film formation was governed by the draining regime.…”

“…Moreover, it was complicated to have a good control on the film formation in water because of the high surface tension of this solvent. 71 Therefore, at high ws, we cannot control the self-assembly. We have observed large and inhomogeneous patches but they were composed of fibrillar domains at pH 4, while aggregates persisted on the surface at pH 6.…”