Irreversible nanogel formation in surfactant solutions by microporous flow

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Entry by Max Darnell, AP 225, Fall 2011

Reference:

Title: Irreversible nanogel formation in surfactant solutions by microporous flow

Authors: Mukund Vasudevan Eric Buse, Donglai Lu, Hare Krishna, Ramki Kalyanaraman, Amy Q. Shen, Bamin Khomami, and Radhakrishna Sureshkumar

Journal: Nature Materials, March 21, 2010

Keywords: micelle, shear-thickening, nanoporous gel, flow-induced structure

Summary

The production of nanostructured materials, especially those with biological applications has recently received much attention. One area of significant interest is the formation of micelles by surfactant molecules. In the past, flow-induced structure (FIS) was the means of creating such micelles, the mechanism of which is shear thickening of the fluid. One drawback, however, is that once flow ceases, these micelles dissociate. As opposed to other micelles, these FIS micelles are anisotropic and aggregate to form gels, due to the directionality of the flow. For this study, cetyl-trimethyl ammonium bromide (CTAB), a well-characterized surfactant that, in the presence of sodium salicylate (NaSal), is known to become structured at high shear rates, was used. This paper outlines a microfluidic method by which these FIS gels can be produced irreversibly and this used in practice outside regions of flow.


Methods/Results

First, standard rheometry using a cone-and-plate rheometer was carried out as to characterize the shear-thickening behovior of the CTAB/NaSal solution that governs the micelle formation. In Figure 1a) below, the shear-thickening nature of the solution is clearly visible, since at a critical shear stress, the apparent viscocity of the solution jumps considerably. Figure 1b) shows that the time required for FIS micelle formation decreases as shear stress increases. A proposed mechanism for why the micelles dissociate when the flow is stopped is that under flow conditions, the collisions between polymers amount to an attractive force and actually overpower the inherent thermal and electrostatic interactions that would otherwise keep the particles apart. Without flow, however, the electrostatic and thermal interactions once again dominate, leading to dissociation of the micelle.

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The authors then cite a theoretical model by Turner and Cates which suggests that not only is high shear necessary for micelle formation, but that highly-aligned flow is also necessary. Thus, the authors employed a microfluidic setup that contained glass bead, forming a porous network. Therefore, flow through the beads is highly ordered but also is strong enough to induce micelle formation. Figure 2 shows different stages of the gel/micelle formation and its interaction with the packed beads. Long, stable gels can be seen. It should be noted that the authors were able to store such gels for months without seeing degradation. This is a marked improvement over previous methods by which the gels immediately dissociated.

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Finally, the authors characterized these nanogels to ensure that they properties are suitable with the new manufacturing method. CryoTEM was used to observe the nanostructure of the material, which is given in the figure below. It can be seen that the average micelle width was 28nm and that the average inter-micelle spacing was 41nm.

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Connection to Soft Matter

Gels are a very common focus of research in soft matter physics due to their applications in biomedicine, photonics, and sensing. The properties of these gels, however, are often dificult to tailor and require caustic chemicals that prevent their use in biological systems. Many of the current biomaterials used in biomedicine are micro or even macroporous, and the space of nanoporous gels has not been deeply explored. This paper's use of purely physical means to create robust nanoporous gels suggests the potential viability of using nanogels in biological applications. From a manufacturing perspective, this paper highlights one of the more successful broad applications of microfluidics, that being the ability to capitalize on low Reynolds-Number flows to manipulate structures on scales that otherwise were only tractable via chemical means.