Scientists have observed that the presence of a solute concentration gradient in solvents will not generate an appreciable volume flow. This phenomenon is not the same when two solutions differing in concentration are separated by a semipermeable membrane. In the second case, it is observed that the solvent at the side of lower concentration passes through the membrane diluting the solution of higher concentration.
A vesicle is a small membrane-enclosed sack that can store or transport substances. When it is placed in a solution with a solute concentration gradient, one side of the vesicle has a higher solute concentration than the opposite side. Thus, the osmotic driving force causes solvent to cross the vesicle’s membrane from inside to outside at high concentration end, and from outside to inside at the low concentration end. Such behavior acts as a microengine, sucking in fluid on one side and ejecting fluid on the other. This movement is known as osmophoresis and it allows the vesicle to reach regions of low concentration. Osmophoretic motion could play some role in the motility of cells and it can be applied in the targeting of agents toward specific regions.
Two scientists from the National Taiwan University presented a theoretical study for the osmophoretic motion of a spherical vesicle in a solution located between two infinite parallel plane walls. This analysis is performed in the limit of negligible Reynolds and Peclet numbers. The applied solute concentration gradient is uniform and parallel to the two plane walls. Also, the vesicle velocity can cause two basic effects due to the surrounding walls. First, the vesicle can speed up or slow down because of alterations in the local concentrations on both sides of the vesicle surface. Secondly, the walls enhance the viscous interaction effect on the moving vesicle. Mathematical analysis is used to generate equations with general solutions in both the rectangular and the spherical coordinate systems. The boundary conditions are enforced first at the plane walls by the Fourier transforms and then on the vesicle surface by a collocation technique.
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2. Chen, P. and Keh, H. Boundary effects on osmophoresis: motion of a spherical
vesicle parallel to two plane walls. Chemical Engineering Science 58 (2003)