MRI: Development of Instrumentation for Direct Measurement of Forces Between a Colloidal Particle and a Surface

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CTS-9977459Dickinson, Richard BAbstractDevelopment of Instrumentation for Direct Measurement of Forces Between a Colloidal Particle and a Surface The interaction forces between particles and surfaces are of central importance to several current industry and biomedically relevant research projects at the University of Florida (UF). These include research efforts in the NSF Engineering Research Center for Particle Science and Technology (ERC) that focus on relating interparticle forces to dispersion and flocculation of particle suspensions. Of particular interest in this ERC is the role of polymer-induced colloidal forces such as steric and bridging interactions. The ERC is also investigating the role of particle surface properties in particle flocculation and deposition to surfaces in efforts to develop new and enhance existing particle separation processes. In other efforts at UF, the mechanism of bacterial attachment to biomaterial surfaces are being investigated in order to develop fundamental understanding the role of specific binding of cell surface macromolecule in the process of attachment. In other cell adhesion research, a collaborative seed project in the Biomedical Engineering Program funded by the Whitaker Foundation is investigating the physicochemical basis for specific cell adhesion to engineered biomaterial surfaces. Each of these current projects and similar envisioned projects would be greatly aided by the ability to directly measure the dynamic interaction forces between colloidal particle and a test surface. Furthermore, successful development of instrumentation to do so would have a broad impact on similar efforts elsewhere.An instrument that is capable of directly measuring static and dynamic forces between a single colloidal particle and a test surface. The instrument consists of a single-beam gradient laser trap (optical trap), which is used to micromanipulate the colloidal particle near the test surface. The particle position is precisely measured from the light scattered from an evanescent wave at the test surface. The force between the particle and the surface is measured from the deflection of the particle from the trap center. Scanning the trap position toward the surface allows measurement of the static interaction force as a function of separation distance. Furthermore, by analyzing the Brownian fluctuations of the particle position, more complex dynamic forces such asviscoelastic polymer-induced forces can be measured as a function of separation distance. To enhance this capability, a piezoelectric device will oscillate the optical trap at desired waveforms to allow analysis of the forces in response to an applied forcing function. The main novelty of this approach is to use the single beam gradient trap as a force transducer, analogous to a cantilever in atomic force microscopy, in combination with evanescent wave light scattering, which precisely measures the particle separation distance form the surface.A prototype of the instrument has been developed and has successfully demonstrated the proof-of-concept. The prototype is capable of simultaneously and accurately measuring static and dynamic forces vs. separation distance between an approximately one-micron colloidal particle and a transparent, optically flat surface. The sensitivity of the prototype ranges between 0.01 and ~5picoNewtons at ~ 1-3 nm spatial resolution. In terms of sensitivity, measurable particle sizes, and spatial resolution, this instrument has significant advantages over other available techniques for direct force measurement on colloidal particles. Based on the successful performance of the prototype, it is planned to develop a more sophisticated instrument for ultimate systematic use by multiple investigators in multidisciplinary research projects at UF. The proposed instrument will include a much more powerful trapping laser and a piezoelectric position device for more dynamic control on the position of the optical trap. Also, it will be capable of measuring backscattered light form the trapping laser, which will allow detection of submicroscopic particles in the trap and a means to calibrate the trap far from the surface. Cooled photomultiplier tubes will enhance the signal-to-noise ratio and provide more precise measurements. These features will greatly enhance the performance capabilities over the prototype.