Initial Investigations of Self-organization in Driven Colloidal Model Systems

Abstract

All systems exhibit the unique phenomenon of self-organization, whether in living or non-living systems. The underlying mechanism of transitioning from a nonordered to an ordered state poses significant questions yet to be answered. The complexity of studying the states of self-organization increases in a living system compared to artificial systems, as its entities of all sizes can perform locomotion. However, because of that locomotion, interesting patterns are observed at different states of self-organization arising from the collective behavior of moving constituents. From the statistical physics point of view, such systems exhibit a state of non-equilibrium; hence, they are known as active matter, and their subconstituents are referred to as active particles. Using objects of a micron size can significantly fill in the gaps of obtaining an ordered system as it could be easily recorded through standard video microscopy for extended periods. One could employ biological motile active matter. While such systems can be harnessed easily from the environment and are used to obtain rich insights into swimming mechanisms at low Reynolds numbers, obtaining proper control over the system is still challenging. Another approach is to design a system consisting of microsized entities that can be triggered to move by external factors at high precision. Such systems can be obtained through active or passive colloidal particles. This work tackles self-organization and pattern formation in both systems of colloidal particles. In the first part, Janus particles were used as active colloids. The design of the Janus colloids consisted of polymeric microspherical particles with hemisphere caps of thin layers of Co and Pd metals. The Co element in the caps provides a magnetic response in external magnetic fields, while the Pd element acts as a catalyst for hydrogen peroxide decomposition. First and foremost, the sensitivity of Janus colloids to external magnetic fields was investigated through fluctuations of the polar angle of their caps and the response time to applied in-plane magnetic fields with varying strengths. The results showed the particle’s response time was faster upon increasing the magnetic strengths while the caps’ fluctuations were reduced. Moreover, due to the magnetic moments of the caps, Janus particles tend to self-assemble over time into larger structures. Here, clusters of dimers and trimers were investigated for their catalytic propulsion and magnetic steering. Each cluster size was classified according to the cap-cap configuration, and each showed a distinguished actuating regime. The second part investigated lane formation and transport of passive colloids. Here, superparamagnetic particles with a 4.5 μm diameter were used as passive colloids, where the interparticle distances were tuned through perpendicular magnetic fields. The particles were driven through gravitational forces by inclining the experimental setup. The resistance of particle transport through 30-μm-wide microfluidic channels with incorporated obstacles was recorded as a function of tilt angle. The obstacles were designed as double U-shaped step-like barriers distanced in the order of a few interparticle distances. Initial records showed three transport regimes depending on the tilt angles. Additionally, the particles’ velocities regarding the particle position around the barriers were estimated.