How Rigid Rods Self‐Assemble at Curved Surfaces

Abstract

Spherical or low-aspect-ratio particles have been used for producing superstable foams by tuning the particle wettability and interparticle interactions. [1][2][3][4][5][6][7][8][9] What is remarkable about these foams is that the stabilization against disproportionation arises from jamming in the adsorbed particle layer at a bubble surface, [10][11][12][13] rather than solidification or gelling of the continuous phase. Realizing the importance of particle shape, Alargova et al. [10,14,15] have developed a novel method for the preparation of flexible high-aspect-ratio microrods from epoxy resin; these rods were able to produce foams with excellent stability. Air bubbles stabilized by these rods have wool-ball or hairlike structures, in which rods are bended and entangled at the bubble surfaces.[10] When looking at these images an interesting question arises-what would happen if the rods become rigid and how would such rigid rods selfassemble at curved substrates, especially when the curvature and rod length are comparable?To investigate this phenomenon, we used rigid CaCO 3 rodlike particles with a monomodal distribution in both diameter (0.5-1.0 mm) and length (10-30 mm; see Figure S1a in the Supporting Information). We chose the rod length to be comparable with the diameter of the smaller bubbles generated upon manual shaking, which allowed us to observe the self-assembly of these rigid rods on both large and small bubbles. Commercially available CaCO 3 rods are hydrophilic and have negligible surface affinity and foamability. We altered the particle hydrophobicity by modifying their surfaces with oleic acid [16] (see Figure S1b and S1c in the Supporting Information), which was expected to result in improved foaming properties. [3,10] However, dispersions of these modified CaCO 3 rods in distilled water produced hardly any foam, while quick aggregation and precipitation of the rods in the cylinder were observed. When ethanol (160 mm, 0.75 wt %) was added to these dispersions, they produced very fine foams with an excess of 50 vol % of air. These foams showed an excellent stability of longer than four months (see Figure S2a and S2b in the Supporting Information), which is comparable with that of other particle-stabilized foams. [4,6,10] The addition of a larger amount (10-90 vol %) of ethanol to particle dispersions has been previously used to change the solvent quality and improve particle wettability, foaming, and emulsification capacity. [3,17] However, in our case, the contact angle of modified rods against a solution of ethanol in water (2.0 wt %) was practically the same as that against distilled water, which indicated that the wettability of modified rods was hardly influenced by the presence of such a small amount of ethanol. We hypothesized that the role of ethanol is to switch off an effective sticky interaction between modified CaCO 3 rods, by partitioning into the fatty acid layer at the particle surface and improving its swelling in water. This in turn can lead to a weak short-range steric repulsion...

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