(476b) Capillary Mode Transitions in Micro-Disc Assembly at Fluidic Interfaces

Capillarity-induced self-assembly at fluid/fluid interface is critical to the design and fabrication of large-scale complex structures (i.e., colloidosomes, pickering emulsions, and foams). Microscale and mesoscale particles with high horizontal-to-vertical aspect ratios have recently drawn attention as building blocks of self-assembled structures thanks to their well-defined curvature fields at the interface. Nevertheless, the underlying dynamics of the lateral capillary interaction between particles at such length scales are still not thoroughly understood.

Herein, we present experimental evidence that unveils the effects of the geometric, hydrophobic, and undulated contact line-induced attractive potential to describe the interfacial interactions and capillarity regimes between sub-millimeter disk-shaped micro-particles at different fluidic interfaces; we systematically reduce the lateral dimensions of the micro-disks and experimentally probe the predictive size limit of the classical two-dimensional theoretical treatment for lateral capillarity, and demonstrate the geometric and hydrophobic regimes where attractive interactions that stem from topographical fluctuation-induced meniscus undulations (quadrupolar capillarity) outstrip the capillary interactions that arise out of interfacial distortion created by gravitational force and hydrophobicity of the particles (monopolar capillarity). This newly identified design parameter can be used to control the assembly of two-dimensional hierarchical super-structures that exhibit phase mixing or phase separation. These insights are important for guiding system design as researchers steadily downscale the dimensions of constitutive parts and leverage lateral capillarity for materials assembly at fluidic interfaces.