Mesoscale Self-Assembly: Capillary Interactions When Positive and Negative Menisci Have Similar Amplitudes

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Original entry: Sorell Massenburg, APPHY 226, Spring 2009

Jeremy L. Steinbacher,Rebecca W. Y. Moy, Kristin E. Price, Meredith A. Cummings, Chandrani Roychowdhury, Jarrod J. Buffy, William L. Olbricht, Michael Haaf, and D. Tyler McQuade. J. Am. Chem. Soc., 2006, 128 (29), pp 9442–9447

Soft Matter Keywords

Capillary force, PDMS, hydrophilicicity


Soft Matters

The authors are following up on earlier work of self assembling hexagonal plates via capillary force: Self-Assembly of Hexagonal Rods Based on Capillary Forces and Mesoscale Self-Assembly of Hexagonal Plates Using Lateral Capillary Forces: Synthesis Using the “Capillary Bond”

Here as in one of the previous papers, the authors fabricate PDMS hexagonal plates doped with aluminum. Unlike before, the authors now vary amount of alumina powder added to achieve a density ranging between 1.05<math>g/cm^3</math> and 1.86<math>g/cm^3</math> to control vertical location of the plates with respect to the interface. The plates were located at the interface between <math>H_2O</math> and perfluorodecalin (relative density: 1.91). Previously, it was found that such plates will aggregate into well ordered structures evocative of the hydrophilicity of each edge. In this work, varying relative density changes the amplitude of the meniscus. Understanding assembly in light of this variance motivated this work.

Relative strength of capillary force as function of density influenced mensici (thick black lines indicate hydrophobicity and thin indicate hydrophilicity.

At relative density of 1.05 (no alumina) the plates' positive menisci are much larger than the negative. There is a strong force between positive menisci and weak capillary forces between negative menisci. At the highest relative density, 1.86, the plates see the reverse (postive menisci much smaller than negative). At a more moderate relative density between 1.15 and 1.75, the amplitudes of the two menisci types are roughly equal. In this regime, there is a strong force between two positive menisci and between negative menisci.

The authors placed 80 hexagonal plates at the interface and then shook the solution (at 1.2Hz). They the mapped the structures formed at a given density and hydrophillic configuration.

Summary of hexagonal hydrophillic configurations used in experiments.
Summary of observed interactions.