Self-Assembly of Microscale Objects at a Liquid/Liquid Interface through Lateral Capillary Forces

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Original Entry: Ian Burgess Fall 2009


Reference

N. Bowden, F. Arias, T. Deng, G.M. Whitesides, "Self-Assembly of Microscale Objects at a Liquid/Liquid Interface through Lateral Capillary Forces", Langmuir 17, 1757-1765 (2001).


Summary

This paper studied the self-assembly of millimeter and micrometer-sized hexagonal polymeric plates with faces patterned into hydrophobic and hydrophilic regions, due to capillary forces at a perfluorodecalin/water interface. The arrays that assembled from the micron-sized objects were were compared to those from millimeter-sized objects. While significant for the millimeter-sized particles, buoyancy forces were found to be negligible in comparison to the vertical capillary forces for the micron-sized particles. While the types of assemblies were similar for the two different scales, more defects were found with the smaller particles. The authors hypothesize that this is because the strength ratio of the capillary:shear forces increases with decreasing particle size, with the ideal balance between attractive capillary forces and repulsive shear forces reaching the ideal balance at a larger size. This paper also details the fabrication technique required to fabricate micron-scale hexagons with selective hydrophilic/hydrophobic edge combinations.

Soft-Matter discussion

This paper probes the effects of capillary forces on objects at the interface between water and a hydrophobic liquid (perfluorodecalin). The forces are probed by coating different faces of hexagonal microparticles with either hydrophobic or hydrophilic substances. The Figure on the right shows the different menisci and forces that come about for hydrophilic and hydrophobic particles (c), with positive menisci forming for hydrophobic particles and negative menisci froming with hydrophilic particles. Lateral capillary forces are attractive for particles having like menisci and repulsive for particles with opposite meniscus-types (b). Part (d) shows how engineering hybrid hydrophilic/hydrophobic hybrid coatings can cause the particles to be tilted at the interface.

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A second pertinent soft-matter component of this study is an examination of the size-dependence of the relative strength of capillary forces compared to other types of forces in the system such as buoyancy. For larger particles (Fig. (c)), higher buoyancy forces result in smaller meniscus formation. Shear forces were also higher compared to capillary forces for large particles and this allowed improperly packed particles to escape the assembly, causing a net lower defect ratio. For the smaller particles, capillary forces became the dominant force and allowed improperly packed particles to remain bound to the assembly due to the overpowering attractive forces.