Influence of Bénard-Marangoni instability on the morphology of drying colloidal films

Formation mechanism and morphology control of cracks in PEMFC catalyst layer during fabrication process: A review

The catalyst layer (CL) is susceptible to cracking during the fabrication process, which presents challenges to the performance and durability of proton exchange membrane fuel cell (PEMFC). This review systematically cascades mechanisms, factors, methods, and applications to provide the first all-encompassing analysis of CL cracking. To construct a research framework, this review comprehensively analyzes the formation mechanism of CL cracks and outlines various approaches for crack morphology optimization. By combining linear elastic fracture mechanics (LEFM) and related research on the drying of colloidal films, the causes of CL cracks can be attributed to structural defects and stress concentrations. On this basis, the means of crack regulation are illustrated from the perspective of ink components and drying conditions. In the end, the impact of cracks on the performance of CL is analyzed and some novel crack inhibition techniques are introduced. Although this review organizes and summarizes the results of related research, there is still a gap in the field of CL crack research. This is evidenced by the lack of a more accurate mechanism for CL crack formation, the unclarity on the effect of crack morphology on CL performance, and the fact that methods to regulate cracking by changing the drying pattern have yet to be further investigated.

Regulable crack patterns for the fabrication of high-performance transparent EMI shielding windows

Crack pattern-based metal grid film is an ideal candidate material for transparent electromagnetic interference shielding optical windows. However, achieving crack patterns with narrow grid spacing, small wire width, and high connectivity remains challenging. Herein, an aqueous acrylic colloidal dispersion was developed as a crack precursor for preparing crack patterns. The ratio of hard monomers in the precursor, the coating thickness, and the drying mediation strategy were systematically varied to control the spacing and width of the crack patterns. The resulting dense and narrow crack patterns served as sacrificial templates for the fabrication of patterning metal grid films on transparent substrates, intended for optoelectronic applications. These films demonstrated excellent optoelectronic properties (82.7% transmission at 550 nm visible light, sheet resistance 4.1 Ω/sq) and strong EMI shielding effectiveness (average shielding effectiveness 33.6 dB at 1-18 GHz), showcasing their potential as a scalable and effective transparent EMI shielding solution.

Crack densification in drying colloidal suspensions

As sessile drops of aqueous colloidal suspensions dry, a close-packed particle deposit forms that grows from the edge of the drop toward the center. To compensate for evaporation over the solid's surface, water flows radially through the deposit, generating a negative pore pressure in the deposit associated with tensile drying stresses that induce the formation of cracks. As these stresses increase during drying, existing cracks propagate and additional cracks form, until the crack density eventually saturates. We rationalize the dynamics of crack propagation and crack densification with a local energy balance between the elastic energy released by the crack, the energetic cost of fracture, and the elastic energy released by previously formed cracks. We show that the final spacing between radial cracks is proportional to the local thickness of the deposit, while the aspect ratio of the crack segments depends on the shape of the deposit.

Criteria for Crack Formation and Air Invasion in Drying Colloidal Suspensions

The drying of sessile drops of aqueous colloidal suspensions leads to the formation of a close-packed particle deposit. As water evaporates, a solidification front propagates from the edge of the drop toward the center, leaving behind a thin disk-shaped deposit. For drops with sufficiently large particle volume fractions, the deposit eventually covers the entire wetted area. In this regime, the dynamics of the deposit growth is governed by volume conservation across a large range of particle volume fractions and drying times. During drying, water flows radially through the deposit to compensate for evaporation over the solid's surface, creating a negative pore pressure in the deposit which we rationalize with a hydrodynamic model. We show that the pressure inside the deposit controls both the onset of crack formation and the onset of air invasion. Two distinct regimes of air invasion occur, which we can account for using the same model that further provides a quantitative criterion for the crossover between the two regimes.

Cracking of Colloidal Films to Generate Rectangular Fragments

Cracks are common in nature. Cracking is known as an irreversible and uncontrollable process. To control the cracking patterns, many researchers have proposed methods to prepare notches for stress localization on films. In this work, we investigate a method of controlling cracks by making microscale pyramid patterns that have notches between the pyramids. After preparing pyramid patterns consisting of colloidal particles with organic residue, we annealed them to induce volume shrinkage and cracking between the pyramids. We studied the effect of film thickness on cracking and the generation of rectangular fragments consisting of multiple pyramids. The area of rectangular fragments was in good agreement with the results of scaling analysis. The concept of controlling cracks by imprinting notches on a film and the relationship with the film thickness can guide the study of cracking phenomena.

Patterning of a Drying Emulsion Film

Stabilizing layers of colloidal dispersions or emulsions to obtain homogeneous films is a real challenge. We describe here a new kind of instability in drying films of emulsions: during evaporation of the internal phase, cracks appear between the droplets that create aggregates according to a regular pattern. We show that this pattern only appears if the emulsion is adhesive, i.e., if droplets stick together. The pattern exhibits a characteristic length which depends on the adhesion strength and film thickness. These experimental results support a model where this instability is due to the gel structure and elastic properties of adhesive emulsions. Understanding this phenomenon will allow us to get a homogeneous film or to control it to get structured materials.

Coupling between wetting dynamics, Marangoni vortices, and localized hot cells in drops of volatile binary solutions

HYPOTHESIS

A sessile drop comprising a mixture of volatile solvents supports spatial variations in interfacial energy, which gives rise to solutal Marangoni flow, alongside evaporative loss of drop mass. Both the Marangoni flow and evaporation bring about a dance of concurrent and inter-connected phenomena: internal Marangoni vortices, localized hot cells, and complex wetting dynamics.

EXPERIMENT

We employ Particle Image Velocimetry and Infra-Red Microscopy to visualize Marangoni vortices, temperature variations, and the wetting dynamics of drops of toluene and ethanol mixtures.

FINDINGS

The intensity of the measured phenomena vary concurrently in time and in like manner according with the initial composition of drops. In particular, we observe maximum intensity levels when the initial toluene proportion in the drops is 60%, and none of these phenomena in the case of pure toluene. Moreover, the drops initially expand on the solid in response to Marangoni flow, then contract due to evaporation; between these dynamic wetting regimes, we further observe a regime of one or periodic wetting/de-wetting cycles at low toluene concentrations. Our findings indicate that both the solutal Marangoni flow and evaporation drive the different phenomena we observe and confirm the connection between Marangoni vortices and the formation of localized hot cells.

Cracking to curling transition in drying colloidal films

Drying-induced cracking is widely encountered in nature and is of fundamental interest in industrial applications. During desiccation, the evolution of water content is nonlinear. Considering the inhomogeneous procedure of desiccation, it is worth considering whether water content will affect the crack pattern formation. To address this concern, in this paper, we report an experimental investigation on the effect of water content on the failure mode in drying colloidal films. A distinct failure transition from random cracking to curling is found when the initial water content increases gradually. When the water content is below a critical value for given film thickness, random desiccation cracking driven by shrinkage is observed. Beyond this critical water content, the film curls with the advent of several main cracks. It is also found that the critical water content corresponding to the transition point depends on the film thickness. In order to qualitatively interpret the experimental observation, a theoretical model is established by adopting the fracture mechanics based on the energy method. The model is found to agree well with the experimental results, elucidating the effects of initial water content on the crack patterns and the transition of failure modes.

Narrowing Desiccating Crack Patterns by an Azeotropic Solvent for the Fabrication of Nanomesh Electrodes

Desiccation of a colloidal layer produces crack patterns because of stress arising out of solvent evaporation. Associated with it is the rearrangement of particles, while adhesion to the substrate resists such movements. The nature of solvent, which is often overlooked, plays a key role in the process as it dictates evaporation and wetting properties of the colloidal film. Herein, we study the crack formation process by using a mixture of solvents, water, and isopropyl alcohol (IPA). Among the various ratios, a water/IPA mixture (15:85 by volume) close to the azeotropic composition possesses unusual evaporation and wetting properties, leading to narrower cracks with widths down to ∼162 nm, uncommon among the known crackle patterns. The dense and narrow crack patterns have been used as sacrificial templates to obtain metal meshes on transparent substrates for optoelectronic applications.