Brownian Dynamics of Particles
Most stable colloidal dispersion undergo Brownian motion, yet most direct force measurement techniques require immobilized particles or surfaces. We employ total internal reflection microscopy (see below) to probe the fundamental forces acting on particles while subject to Brownian motion. Our current interests included forces in polyelectrolyte solutions and the diffusions of anisotropic particles near surfaces.
Total Internal Reflection Microscopy
TIRM, developed by Prieve et al. (1), is an optical noninvasive experimental technique which measures the interaction potential energy of a levitated colloidal particle undergoing Brownian motion above a flat surface. The particle frustrates an evanescent wave created at the solid liquid interface above the critical angle, as shown in the image. The intensity of the scattered light varies exponentially with the particle elevation. The evanescent wave decay length is on the order of 100 nanometers, which translates into a height resolution on the order of 1 nanometer.
The Brownian motion of the colloidal particle is tracked overtime, tabulated and converted to the interaction potential energy assuming the motion of the particle follows a Boltzmann distribution. Optical tweezers are also employed for solution changes and to bias the heights the particle samples to better elucidate different regions of the interaction energy. The optical tweezers act as a body force which can modulate the effective buoyancy of the particle.
TIRM can also be used to track the hindered diffusion of a particle near an interface by tracking the correlated motion of the pearlite. A range of phenomena have been examined using TIRM including the diffusion of a particle near an interface, depletion force that arise from particles or polymers, retarded van der Waals-Casimir-Lifshitz forces, steric forces and electrical double layer forces.
(1) Prieve, D. C.; Frej, N. A., Langmuir 1990, 6, (2), 396-403
Single Molecule Forces Spectroscopy
Using atomic force microscopy (AFM) our group has studied the behaviors the unfolding or extension of individual polymer or protein molecules. An exciting example of this type of research has been in collaboration with Prof Richard Wetherbee in the School of Botany, where we probed the protein structure in biofouling films from diatoms and algae. Using AFM, we were able to quantify the protein structure through the features observed during the protein extensions. Using AFM to learn about proteins that act as bioadhesives (such as muscle proteins or spider silk) has been accomplished previously, but this work was the first to achieve this on a living biological organism. Understanding the molecular structure of these proteins and bioadhesives will help in the design of antifouling coatings for marine vessels.