Role of external forces and additives on the desiccation patterns of colloidal droplets

Abstract

The self-assembly of dried colloidal droplets has emerged as a critical area of research due to its wide applicability in coatings, printing, and functional materials. Over the years, this phe nomenon has been extensively studied for smooth surfaces with spherical nanoparticles, focusing on evaporation-driven deposition and pattern formation. However, the behavior of particles or substrates with complex topographies remains relatively unexplored, despite its potential to un lock new functionalities and applications. This thesis explores the intricate dynamics of particle deposition and crack formation during the drying of colloidal droplets, focusing on the interplay between surface morphology, particle characteristics, and external stimuli such as magnetic fields and polymer additives. We begin by investigating the influence of surface roughness on droplet evaporation and deposition patterns, comparing self-affine and corrugated nanorough surfaces. Our findings reveal anisotropic wettability and irregular dried patterns on corrugated surfaces, highlighting the role of surface structuring in controlling droplet dynamics. Next, we address the challenge of achieving uniform particle distribution and crack suppression by incorporating water-soluble polymers like poly(vinyl alcohol) into colloidal suspensions. The addition of poly mer alters fluid flow dynamics, enabling a transition from ring-like deposits to uniform coatings with reduced cracking, offering a practical solution for crack-free coatings. Further, we explore the alignment of anisotropic nanoparticles under inclined magnetic fields, demonstrating how magnetic field orientation influences crack patterns and particle self-assembly. Our experiments reveal unique crack morphologies, including hook-shaped and helical patterns, governed by the interplay between magnetic torque and hydrodynamic forces. Finally, we examine the impact of ferro-colloidal particle size and concentration on crack propagation, showing that magnetic field direction and particle alignment significantly affect deposit integrity and crack density. Col lectively, this work provides fundamental insights into the mechanisms governing evaporative self-assembly, crack formation, and particle alignment in colloidal systems. By leveraging sur face engineering, polymer additives, and external magnetic fields, we develop novel strategies for tailoring deposition patterns and controlling crack morphology. These findings have broad impli cations for advancing technologies in inkjet printing, microfabrication, and functional coatings, paving the way for innovative applications in materials science and nanotechnology.