Viscosity of Picoliter Volumes Measured by Nondegenerate Two-Wave Mixing 6. AUTHORS

Using nondegenerate two-wave mixing in colloidal suspensions we measure the viscosity of liquids with as little as 20 picoliters of volume. In addition, changes in the frequency signature with intensity are observed which are interpreted in terms of selffocusing effects. Typical viscosity measurements require volumes ranging from tens of milliliters to liters of liquid. For many applications in biology, chemistry, and medicine where there may only be very small amounts available, this is impractical. We have developed a technique which measures the absolute viscosity of solutions to within ten percent of previously tabulated values with sample volumes as small as 20 picoliters. This technique relies on the forces associated with radiation pressure. The technique is accurate and not difficult to perform or interpret and is based on the nonlinear optical effect of nondegenerate two-wave mixing (NDTWM). The systems under consideration are solvents containing suspensions of polystyrene microspheres. In the experiments we performed, the solvents under consideration were mixtures of water and glycerol. A dielectric particle within the solution of interest will feel a force exerted on it in a light field with a strong intensity gradient in the direction of the gradient. The force is given by >>: