(188e) Near Field Capillary Interactions Between Microparticles with Undulated Contact Lines

Microparticles at fluid interfaces interact by capillarity.  Far from contact, particles are known to attract and interact as quadrupoles in the interface.  In the near field, geometric details dominate interactions. In practice, particles or particle agglomerates can have highly irregular surfaces that can create undulated interfaces in the near field.  The interactions owing to these undulations may be repulsive or attractive (J. Lucassen, Colloids Surfaces, 65 (1992), pp. 131–137). Here we present a fundamental study between particles with undulated interfaces in the near field.

We have designed and fabricated microparticles with corrugated edges in the form of sine waves.  When placed at interfaces, the corrugations pin the contact line, creating undulations in the interface with wavelengths and amplitudes dictated by the particle shape.  Far from contact on unbounded interfaces, the particles interact like the expected capillary quadrupoles.  In the near field, interactions depend on the details of the near-field undulations. Identical microparticles assemble with their corrugated edges in phase. Those with the same wavelengths and amplitude but out of phase corrugations shift to align their features.    Microparticles with different wavelengths but the same amplitude rotate weakly to form oblique assemblies.  To study the interactions of aligned undulated features, we confine the microparticles to an interface in a channel that is slightly wider than the dimension of the microparticles. For non-identical particles, we report an equilibrium separation distance due to near-field capillary repulsion.