Osmotically driven shape-dependent colloidal separations

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by Lidiya Mishchenko


Mason, T.G. Physical Review E 66, 060402(R) 2002


Osmotic pressure, entropy, assembly, micelle, colloid, excluded volume


"The thermally induced motion of nanometer-sized surfactant micelles in water is used to create strong attractive forces between micron-sized disks of wax in a mixed aqueous dispersion of microdisks and microspheres. The short-ranged attractive force due to the depletion of micelles from between the microdisks is much stronger than that between two microspheres of similar size, and is largest when the disks approach face to face, so columns of microdisks form. These columns cream, whereas the spheres remain dispersed, providing a means for shape-dependent colloidal separations driven by an applied micellar osmotic pressure."

Soft Matter Example

This paper utilizes entropically driven assembly first proposed by Asakura and Oosawa in 1954 ("On interaction between two bodies immersed in a solution of macromolecules"). The basic idea is as follows: A mixture of large and small spheres leads to assembly of the larger spheres. This is because these large spheres create area of "excluded volume" around them which small spheres cannot enter (with hard sphere interactions). When two large spheres come into contact, however, they reduce the excluded volume of the smaller spheres, thus increasing the entropy of the system. Another way of understanding this "attractive" force that develops between large spheres is that when they approach each other and exclude small particles between them, the "nonuniform effective pressure of the smaller spheres over the larger spheres creates a net attractive force" (i.e. there's an uncompensated force from the smaller spheres on the outer sides of larger approaching spheres). This is very similar to the driving force in osmosis and is thus called "osmotic pressure".

Excluded volume (shown in yellow) between particles of different geometries. Purple particles are micelles (not to scale)