Effects of Substrate Constraint on Crack Pattern Formation in Thin Films of Colloidal Polystyrene Particles

Abstract

Crack formation and the evolution of stress in drying films of colloidal particles were studied using optical microscopy and a modified cantilever deflection technique, respectively. Drying experiments were performed using polystyrene particles with diameters of 47 ± 10 nm, 100 ± 16 nm, and 274 ± 44 nm that were suspended in water. As the films dried, cracks with a well-defined spacing were observed to form. The crack spacing was found to be independent of the particle size used, but to increase with the film thickness. The characteristic crack spacing was found to vary between 20 and 300 μm for films with thickness values in the range 3-70 μm. Cantilever deflection measurements revealed that the stresses that develop in the film increase with decreasing film thickness (increasing surface-to-volume ratio). The latter observation was interpreted in terms of the effects of a substrate constraint which causes the build up of stresses in the films. This interpretation was confirmed by crack formation experiments that were performed on liquid mercury surfaces in which removal of the substrate constraint prevented crack formation. Experiments were also performed on compliant elastomer surfaces in which the level of constraint was varied by changing the substrate modulus. The cracking length scale was found to increase with decreasing substrate modulus. A simple theory was also developed to describe the substrate modulus dependence of the cracking length scale. These combined experiments and theory provide convincing evidence that substrate constraints are an important factor in driving crack formation in thin colloidal films.

Colloidal processing: enabling complex shaped ceramics with unique multiscale structures

“…Also, the mechanisms that control the resistance to fracture of the wet particulate body are not known. The study of the spacing between cracks has also been a recent interest …”

Effect of Surface Wettability on Crack Dynamics and Morphology of Colloidal Films

“…26,27 The crack formation mechanism is a complex function of several parameters such as particle size, 28 charge, 29 shear modulus, 30 and process parameters such as evaporation rate. 31 The manipulation of substrate properties 32 to control the crack formation process has recently gained importance. The aim is to have higher critical cracking thicknesses, and several studies report innovative approaches to achieve the same.…”

The interplay of crack hopping, delamination and interface failure in drying nanoparticle films

“…A little further behind the compaction front, the film is observed to form cracks that propagate parallel to the drying direction. These cracks form as a result of stresses that build up in the film due to substrate constraints, which confine the drying film and leave it in a state of tension89. Dufresne et al 17 observed similar cracks to the ones shown in Fig.…”

“…The complexity of the drying process and the stress distributions that build up in these coatings have meant that providing an accurate theoretical description of the failure mechanisms in colloidal nanoparticle films has been challenging. However, a number of authors have studied how factors such as film thickness23, particle size45, drying rates67 and substrate constraints89 influence the failure mechanisms in thin nanoparticle films. Delamination is one such failure mechanism which can undermine the physical stability of a coating and occurs when a film debonds from its substrate.…”