Browsing by Subject "Brownian motion processes"

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Brownian motion at fast time scales and thermal noise imaging
(2008-12) Huang, Rongxin, 1978-; Florin, Ernst-Ludwig
This dissertation presents experimental studies on Brownian motion at fast time scales, as well as our recent developments in Thermal Noise Imaging which uses thermal motions of microscopic particles for spatial imaging. As thermal motions become increasingly important in the studies of soft condensed matters, the study of Brownian motion is not only of fundamental scientific interest but also has practical applications. Optical tweezers with a fast position-sensitive detector provide high spatial and temporal resolution to study Brownian motion at fast time scales. A novel high bandwidth detector was developed with a temporal resolution of 30 ns and a spatial resolution of 1 °A. With this high bandwidth detector, Brownian motion of a single particle confined in an optical trap was observed at the time scale of the ballistic regime. The hydrodynamic memory effect was fully studied with polystyrene particles of different sizes. We found that the mean square displacements of different sized polystyrene particles collapse into one master curve which is determined by the characteristic time scale of the fluid inertia effect. The particle’s inertia effect was shown for particles of the same size but different densities. For the first time the velocity autocorrelation function for a single particle was shown. We found excellent agreement between our experiments and the hydrodynamic theories that take into account the fluid inertia effect. Brownian motion of a colloidal particle can be used to probe three-dimensional nano structures. This so-called thermal noise imaging (TNI) has been very successful in imaging polymer networks with a resolution of 10 nm. However, TNI is not efficient at micrometer scale scanning since a great portion of image acquisition time is wasted on large vacant volume within polymer networks. Therefore, we invented a method to improve the efficiency of large scale scanning by combining traditional point-to-point scanning to explore large vacant space with thermal noise imaging at the proximity of the object. This method increased the efficiency of thermal noise imaging by more than 40 times. This development should promote wider applications of thermal noise imaging in the studies of soft materials and biological systems.
Contribution of electrostatic interaction to the image formation in 3D thermal noise imaging
(2006-12) Qiu, Jinze; Florin, Ernst-Ludwig
Three dimensional structures are able to be imaged by scanning the volume with a nanometer-size Brownian particle. The contribution of electrostatic interaction to the image formation in thermal noise imaging was studied. The problem was simplified to one dimension by replacing the complex three-dimensional structure with a planar coverslip. A simple fluorescence experiment was designed first to calibrate the distance from the trapping center to the planar surface. Strong electrostatic repulsion between like charged surfaces was observed in the experiments at low ionic strength as expected. Further fluorescence experiments shows that minimum separation decreases as salt concentration increases. It was found that 0.01mol/l is the optimal salt concentration for the given experimental condition. Higher concentrations lead to a permanent adhesion of particles to the surface making thermal noise imaging impossible.
Robert Irwin Guest Lecture
(0000-00-00) Irwin, Robert
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