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In this work, unique functional devices exhibiting controlled gradients of properties are fabricated by dip-coating process in acceleration mode. Through this new approach, thin films with “on-demand” thickness graded profiles at the submillimeter scale are prepared in an easy and versatile way, compatible for large-scale production. The technique is adapted to several relevant materials, including sol–gel dense and mesoporous metal oxides, block copolymers, metal–organic framework colloids, and commercial photoresists. In the first part of the Article, an investigation on the effect of the dip coating speed variation on the thickness profiles is reported together with the critical roles played by the evaporation rate and by the viscosity on the fluid draining-induced film formation. In the second part, dip-coating in acceleration mode is used to induce controlled variation of functionalities by playing on structural, chemical, or dimensional variations in nano- and microsystems. In order to demonstrate the full potentiality and versatility of the technique, original graded functional devices are made including optical interferometry mirrors with bidirectional gradients, one-dimensional photonic crystals with a stop-band gradient, graded microfluidic channels, and wetting gradient to induce droplet motion.

SiO2/TiO2xerogel ultrathin protective coatings, with tunable refractive index between 1.45 and 2.20, have been prepared by dip-coating sol−gel solutions of TiCl4and tetraethyl orthosilicate with various molar compositions. In situ thermal ellipsometry analysis is used to follow the modifications of optical properties of these binary oxide systems upon thermal treatment up to 820 °C. Involved phenomena, such as dehydration, condensation, viscous sintering, crystallization, and diffusive sintering have been clearly identified and studied with respect to the Ti/Si atomic proportion and the treatment conditions. Due to the influence of the crystal size on the semiconducting properties of titania nanoparticles, its crystallization kinetic has been specifically investigated. We show that the crystallization of TiO2is globally taking place between 300 and 600 °C depending on the silica to titania ratio and on the heating rate. Films containing more than 40% of silica remain amorphous at 820 °C. Poor silica contents tend to be associated to lower crystallization temperature (Tc) and larger crystal sizes. The size of the smallest detected anatase nanoparticles was close to 4 nm, while those of the largest were limited by the film thickness around 35 nm. Interestingly, we observed that, while faster heating rates induce high Tcfor pure TiO2, the inverse tendency is observed for binary oxide films, which is explained by the diminution of the atomic diffusion due to the progressive condensation of the silica network. Electronic microscopy investigations revealed that smoother surface topologies and more homogeneously structured films are obtained in the presence of silica.

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−1and temperatures from 25 to 60 °C) and for several different chemical solutions (TiCl4, TEOS, and MTEOS, all in the presence, or not, of block PEO-b-PPO copolymer templates in EtOH/H2O, with concentrations from 10−1to 10−3mol·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.