Structure Formation in Soft‐Matter Solutions Induced by Solvent Evaporation

Abstract

etc., and the interplay between different processes determines the final structure of the dried materials.Here, we report recent advances of selfassembled structures induced by uniform evaporation, examples including drying in a thin-film geometry or evaporating a spherical droplet on a superhydrophobic surface. [4] In these situations, the structure formation takes place in the direction of the evaporation. This excludes the case where lateral flow is induced by nonuniform drying. The lateral flow is important in explaining the coffee-ring effect, where excellent reviews exist in literature. [5,6] After introducing the general procedure in experiments, we explain the physical mechanisms responsible for the structure formation in solvent evaporation. We then discuss recent advances based on single-and binarycomponent solutions, with a special emphasis on the theoretical approaches. Drying of Single-Component SolutionsA general experiment starts with spin-coating a thin-film solution on a substrate, then the sample is left to dry. In the first step, it is important that the spin-cast film is uniform. [7,8] In the second step, the evaporation flux, i.e., the mass removed from the liquid phase to the vapor phase in unit time from unit surface area needs to be controlled precisely. Even for the pure solvent evaporation, the process is complex and involves the diffusion of solvent molecules in both the liquid and vapor phases, and external flow in the vapor phase to prevent saturation. The evaporation flux can be computed using the Hertz-Knudsen equation that is based on the thermodynamics variables [9][10][11] or solving the diffusion equation in the vapor phase using suitable boundary conditions. [12][13][14] The solvent evaporation drives the free surface to recede toward the substrate, resulting in an accumulation of the solutes at the surface. Two competing processes determine the spatial distribution of the solutes. One is the Brownian motion, which is characterized by the diffusion constant D of the solutes. The other is the evaporation, characterized by the rate v ev at which the free surface recedes. Two timescales are related to these two processes: the diffusion time τ D =  2 /D, where  is a characteristic length and the evaporation time τ ev = /v ev . To quantify the relative importance between the diffusion and the evaporation, one can define a dimensionless number called film formation Peclet number: [15]

Colloidal polymer composites: Are nano-fillers always better for improving mechanical properties?

“…In mixtures of colloidal particles with differing sizes, stratification by particle size has been observed, including the observation of a surface layer enriched in small particles. 51 Our research showing how the filler concentration affects the fracture and creep properties could be used in the future to design the surface properties of stratified waterborne coatings.…”

Electrospraying the Triblock Copolymer SEBS: The Effect of Solvent System and the Embedding of Quantum Dots

“…A higher concentration at the surface arises, resulting in a concentration gradient inside the droplet . The time scale of evaporation τ ev = l/v ev in this process is much smaller compared to the timescale of polymer chain diffusion τ D = l 2 / D , so this concentration gradient persists over the lifetime of the droplet, which will result in initially high Peclet numbers Pe = τ D /τ ev = v ev l/ D , where v ev is the evaporation rate, l is a characteristic length, and D is the diffusion coefficient . The form of this concentration gradient and the evolution thereof depends then on the solvent/polymer interaction as well as the concentration of the polymer.…”

Directional freezing of binary colloidal suspensions: a model for size fractionation of graphene oxide

“…[13][14][15][16] In addition, the dilute limit we concern here ensures the validity of the two-body interaction approximation describing the cross-section interaction which has been demonstrated as the crucial factor in controlling the stratification of binary colloids in drying films. 17,18 The remainder of this article is organized as follows: In Section II, we derive the dynamical equations that describe the evolution of binary colloids with a moving freezing front.…”