Microviscosity, also known as microscopic viscosity, is the friction experienced by a single particle undergoing diffusion because of its interaction with its environment at the micrometer length scale. The concept of microviscosity is intimately related to the concept of single particle diffusion and can be measured using

microrheology Microrheology is a technique used to measure the rheological properties of a medium, such as microviscosity, via the measurement of the trajectory of a flow tracer (a micrometre-sized particle). It is a new way of doing rheology, traditionally don ...

. Understanding microviscosity requires an understanding of

viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...

and

diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemica ...

i.e. macroscopic viscosity and bulk diffusion and where their assumptions break down at the micro to nanometer scale where physicists are still trying to replace phenomenological laws with physical laws governing the behavior of single particle mobility. In the field of

biophysics Biophysics is an interdisciplinary science that applies approaches and methods traditionally used in physics to study biological phenomena. Biophysics covers all scales of biological organization, from molecular to organismic and populations. ...

, a typical microviscosity problem is understanding how a biomolecule's mobility is hindered within a cellular compartment which will depend upon many factors such as the size, shape, charge, quantity and density of both the diffusing particle and all members of its environment. Microviscosity can be probed by measuring the

rotational correlation time Rotational correlation time (\tau_c) is the average time it takes for a molecule to rotate one radian. In solution, rotational correlation times are in the order of picoseconds. For example, the \tau_c = 1.7 ps for water, and 100 ps for a pyrroli ...

of a probe molecule using either fluorescence correlation spectroscopy or the linewidths of the probe's

electron spin resonance Electron paramagnetic resonance (EPR) or electron spin resonance (ESR) spectroscopy is a method for studying materials that have unpaired electrons. The basic concepts of EPR are analogous to those of nuclear magnetic resonance (NMR), but the sp ...

. The friction experienced by a single particle can be thought of as a microscopic viscosity (microviscosity) and should not necessarily agree with the bulk viscosity since it is a measure of the probe's local friction whereas bulk viscosity analogously would be the measure of an infinitely large probe. Both the crowding density and relative size of each co-solute in a mixture will contribute to the measured microviscosity as assessed by altered translational mobility.Goins, A.B., H. Sanabria, M. N. Waxham. 2008. Macromolecular crowding and size effects on probe microviscosity. Biophys. J. 95:5362-5373.

References

Soft matter {{CMP-stub