Synopsis: Diffusion in a membrane

Calculations elucidate how the lateral diffusion of macromolecules within a liquid membrane depends on the properties of the solvent layer surrounding it.
Synopsis figure
Carnegie Mellon University/Gordon Rule

Understanding the Brownian motion of macromolecules embedded in a liquid membrane is important because it underlies many fundamental biological and chemical interactions. Calculations have shown that the diffusion coefficients of inclusions moving within the membrane generally increase when the thickness of the solvent on either side of the membrane increases, but this earlier work was limited either to numerical calculations or to certain limiting cases. Now, in Physical Review E, Kazuhiko Seki at the AIST in Tsukuba, Japan, and his colleagues have presented largely analytical results for diffusion coefficients that can take into account solvent layers of arbitrary thickness.

Interactions between the solvent and the membrane exert a drag force, affecting hydrodynamic flow of inclusions in the membrane. This effect may be characterized by a hydrodynamic screening length that depends on the thickness of the solvent. Seki et al. introduce an analytical expression relating diffusion coefficients to these screening lengths. For solvents of finite thickness, the equations yield multiple hydrodynamic screening lengths, together with weight factors. In some cases, only the largest hydrodynamic screening length determines diffusion coefficient, and Seki et al. have tabulated the conditions, such as the membrane and solvent viscosity, the size of the inclusion, and the solvent depth, under which this approximation holds.

This work extends previous calculations of limiting cases and clarifies the limits of their validity. Calculations such as these are becoming increasingly relevant as technological advances make it possible to observe micron-sized objects in membranes. – Margaret Foster


More Announcements »

Subject Areas

Soft MatterBiological PhysicsChemical Physics

Previous Synopsis

Atomic and Molecular Physics

Dirac points multiply in the presence of a BEC

Read More »

Related Articles

Synopsis: Down to Friction
Soft Matter

Synopsis: Down to Friction

Experiments explain the significant increase in viscosity that occurs when stirring a mixture of cornstarch and water. Read More »

Focus: Stalling a Molecular Motor
Biological Physics

Focus: Stalling a Molecular Motor

Researchers used a viscous fluid to disrupt the operation of a molecular motor that transports material inside biological cells, giving insights into how it works.   Read More »

Synopsis: Robust Yet Flexible Clocks
Biological Physics

Synopsis: Robust Yet Flexible Clocks

A theoretical analysis explains why circadian clocks can be robust but also able to adapt to environmental changes.   Read More »

More Articles