Statistics of particle trajectories at short time intervals reveal fN-scale colloidal forces.
We describe and implement a technique for extracting forces from the relaxation of an overdamped thermal system with normal modes. At sufficiently short time intervals, the evolution of a normal mode is well described by a one-dimensional Smoluchowski equation with constant drift velocity v, and diffusion coefficent D. By virtue of fluctuation dissipation, these transport coefficients are simply related to conservative forces, F, acting on the normal mode: F=kBTv/D. This relationship implicitly accounts for hydrodynamic interactions, requires no mechanical calibration, makes no assumptions about the form of conservative forces, and requires no prior knowledge of material properties. We apply this method to measure the electrostatic interactions of polymer microspheres suspended in nonpolar microemulsions.
Department of Mechanical Engineering, Yale University, New Haven, Connecticut 06511, USA.
Sunil K
SK
Sainis
Vincent
V
Germain
Eric R
ER
Dufresne
D003102:Colloids / Q000737:chemistry*; D004058:Diffusion; D012997:Solvents; D055672:Static Electricity; D013997:Time Factors
D003102:Colloids; D012997:Solvents