Amphiphilic Crescent-Moon-Shaped Microparticles Formed by Selective Adsorption of Colloids
Original Entry by Andrew Capulli, AP225 Fall 2011
"Amphiphilic Crescent-Moon-Shaped Microparticles Formed by Selective Adsorption of Colloids" S.-H. Kim, A. Abbaspourrad, and D. A. Weitz. Journal of the American Chemical Society 133(14), 5516-5524 (2011).
There are numerous amphiphilic surfactant molecules available in a wide variety of hydrophilic-hydrophobic balances, shapes, sizes etc as pointed out by the authors. However, as amphiphilic microparticles have become of interest due to their potential use as pigments in display devices and as mentioned by the authors, microparticles can be used as the "building blocks to construct photonic structures through directional interactions." However, surfactant microparticles are of particular interest due to their amphiphilic nature and potential for added increased strength of emulsion. While the authors note that the science behind the creation of such amphiphilic particles is not necessarily novel in this research, the field itself is new and like how their are numerous tunable molecular surfactants (in particular numerous shapes) there is a need in the mircoparticle field for such shape diversity and study into the effects thereof. Dumbbell, mushroom, and spherical shaped amphiphilic microparticles have been studied but there is a need to investigate more and in this methods paper the authors do just that, presenting a new way to form "crescent" (half moon shelled) shaped amphiphilic microparticles.
Summary of Main Experimentation
The paper is essentially a methods paper and the details of the setup are described more thoroughly. Here I summarize them: Weitz et. al. use a microfluidic device (theta injection) to produce their amphiphilic microparticles. Fluorocarbon oil (FC-77) and ethoxylated trimethylolpropane triacrylate (ETPTA) are 'injected' into an aqueous solution via their theta capillary device (see Figure 1 below). Because FC-77 has a higher energy with water than ETPTA, the ETPTA drop will spread around the FC-77 drop. As the authors describe the behavior of the spreading (how much the ETPTA spreads or wets the FC-77 drop) is dependent on the make up of the aqueous solution used as the continuous phase of the set up (blue in Figure 1).
To give some perspective, full spreading is reportedly observed in 3 wt% PVA solution while only partial spreading (more so drop separation) is observed in 1 wt% Pluronic F-108. Essentially, spreading goes as:, where S is the spreading parameter which is dependent on the interfacial tension between phases j and k. The spreading parameter as described by the authors, "reflects the relative values of the surface energies, and the propensity of one fluid to spread on another." If S<0, partial spreading of the ETPTA will occur and if S>0 complete spreading of ETPTA will occur (shelling of the ETPTA over the FC-77). This is shown in figure 2 below.
For the preparation of amphiphilic crescent shaped particles, partial spreading is desired. The ETPTA solution containes hydrophilic silica particles which, once injected into the aqueous solution, will migrate toward the surface of the ETPTA that is in contact with water and away from the surface of the ETPTA that is in contact with the FC-77. This migration is spontaneous because it lowers the energy between the surface of the ETPTA and the water (aqueous solution). By using photopolymerization (UV), the authors polymerize the ETPTA-silca material in its semi-spread (crescent) shape then disperse the FC-77-ETPTA drops in ethanol which allows for FC-77 removal. This is leaves just the crescent shaped polymer with a convex hydrophilic portion due to the presence of silica particles at this surface and a hydrophobic concave portion where the FC-77 once was. Below I have made a hybrid of Figures 3 and Figure 9; Figure 3a below schematically outlines the process just described:
Connection to Soft Matter