Crack control in dried ferro-colloidal droplets: effect of particle aspect-ratio and magnetic field orientations.

Crack formation in dried colloidal films is a common phenomenon encountered in diverse fields, from coatings and materials science to biological and environmental applications. Understanding the mechanisms behind crack patterns and their dependency on external factors is crucial for tailoring deposit structures. In this study, we investigate the impact of an externally directed magnetic field on the crack morphology and self-assembly in dried deposits composed of anisotropically shaped ferro-colloidal particles of varying sizes. Employing a sessile drop configuration, distinct crack patterns are observed in ring-like deposits as the magnetic field is applied in parallel, perpendicular, and oblique orientations. Notably, crack propagation in the oblique field direction transitions from wavy to helical-shaped patterns depending on the size of the nanoparticles, in contrast to the patterns seen in parallel and perpendicular fields. Our findings demonstrate that ferro-colloids align with the magnetic moment along the tensile stress direction, particularly at the edges of the deposits where cracks propagate. The particle orientation and self-assembly in the deposits were controlled by the interaction of hydrodynamic and magnetic forces, with force calculations revealing that this interaction strongly depends on particle size and field angle. This interaction leads to crack alignment along the particle's long axes, emphasizing the influence of the magnetic field on the deposit's structural integrity. Additionally, ferro-colloid concentration significantly impacts crack density, with higher concentrations promoting the development of prominent cracks at the rim edges of the deposits. By leveraging the interplay between magnetic interactions and evaporation dynamics, we can develop novel strategies for manipulating nanoscale structures for advanced technology.