Alireza Mohammad Karim

Experimental study of dynamic contact angles on rough hydrophobic surfaces

Rough hydrophobic surfaces have many applications in industry and technology. An experimental study was done on the spreading dynamics of different concentrations of polyethylene glycol (PEG) solutions on rough Teflon plates with different roughness. The experiments were conducted using Wilhelmy plate method. The advancing dynamic contact angle was found to be weakly dependent of capillary number. However, the receding dynamic contact angle decreases with increasing capillary number. The degree of roughness on rough Teflon surface has an important role on dynamic contact angle. The dynamics of receding motion was found to follow the molecular-kinetic theory. A power law relation between the receding dynamic contact angle and the capillary number was also obtained.

Dynamic self-assembly of charged colloidal strings and walls in simple fluid flows

Colloidal particles can self-assemble into various ordered structures in fluid flows that have potential applications in biomedicine, materials synthesis and encryption. These dynamic processes are also of fundamental interest for probing the general principles of self-assembly under non-equilibrium conditions. Here, we report a simple microfluidic experiment, where charged colloidal particles self-assemble into flow-aligned 1D strings with regular particle spacing near a solid boundary. Using high-speed confocal microscopy, we systematically investigate the influence of flow rates, electrostatics and particle polydispersity on the observed string structures. By studying the detailed dynamics of stable flow-driven particle pairs, we quantitatively characterize interparticle interactions. Based on the results, we construct a simple model that explains the intriguing non-equilibrium self-assembly process. Our study shows that the colloidal strings arise from a delicate balance between attractive hydrodynamic coupling and repulsive electrostatic interaction between particles. Finally, we demonstrate that, with the assistance of transverse electric fields, a similar mechanism also leads to the formation of 2D colloidal walls.