Colloidal Interactions In Field Directed Self Assembly

Author : Pushkar P. Lele
Publisher :
ISBN 13 : 9781124240978
Total Pages : pages
Book Rating : 4.2/5 (49 download)

DOWNLOAD NOW!

Download or read book Colloidal Interactions in Field-directed Self-assembly written by Pushkar P. Lele and published by . This book was released on 2010 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: This thesis discusses: (1) the fabrication of an experimental tool, namely holographic optical tweezers for simultaneously manipulating spatial locations of multiple particles, (2) development of a framework for interpreting hydrodynamic interactions between multiple particles close to a no-slip surface and comparisons of experimental data with predictive modeling results (Stokesian dynamics simulations) (3) investigations of colloidal particle interactions under external AC fields and the intriguing spontaneous pattern formations in the suspension and, (4) the use of an unconventional assemble-stretch technique for creating novel 2D and 3D crystalline arrays of anisotropically shaped particles, from spherical particle templates. By blinking holographic optical traps, we investigate the hydrodynamic interactions in multi-particle ensembles, influenced by a no-slip surface. The measurements are carried out by screening out electrostatic interactions in the suspension. We observe that with increasing proximity with the surface, the effect of particle-particle hydrodynamic interactions on the short-time self-diffusivities is screened. We use the Stokeslet representation of particles and combine it with the method of images to understand the correlated motion of particles within the ensembles. Analysis of the resultant ensemble eigen-modes reveals that even in dilute suspensions, the effective diffusivities decay as the inverse of the separations, over the range of particle-particle separations we experimented with. The relative modes exhibit dominant contributions from close neighboring particles and the collective modes incorporate long-range contributions from all particles in the ensemble. Our analysis also confirms that for larger number of particles in the ensemble, the contributions from particle-particle interactions increase and in concentrated suspensions they over-ride the strong hydrodynamic screening by the wall. We investigate the microstructure of polarizable particles in AC electric fields as a function of field strength and frequency and observe three key features: (1) The order-disorder transition as a function of field strength and frequency exhibits an ordered regime consisting of dipolar chains and crystals between two disordered regimes at low and high field strengths. While the transition to order at low field strengths is consistent with the polarization-induced interactions becoming larger than the thermal energy of the particles, the reentrant disordered regime at high field strengths is marked by large- scale rotating structures; (2) As mentioned, using blinking laser tweezers to characterize the interaction of particle pairs, we demonstrate that particle pair rotation opposite of alignment in the field direction is the microscopic phenomenon driving the observed suspension behavior. The pair particle rotation is observed to decrease with increasing frequency, thus requiring higher field strengths, in agreement with the frequency and field strength dependence of the second order-to-disorder transition; (3) The particle rotation is consistent with a mechanism based on the mutual polarization of particles, and is an effect of the phase lag between the polarization and the applied and mutually induced field. The connection we made between particle interactions and suspension microstructure answers a long-standing debate regarding the mechanism underlying the band structures in experiments employing parallel electrode geometries. Creating ordered crystalline structures from anisotropic particles is a fundamental challenge. To create such structures, either one can direct the assembly of particles from a random dispersion using external fields or, as we will show, they can be created from the controlled deformation of isotropic particle templates. Using an assemble-stretch technique we create 2D and 3D anisotropic particle assemblies from an initial template of close-packed spheres. This method enables the fabrication of anisotropic crystals from particles of any deformable material with a reasonable glass transition temperature, and thus, is a key advantage of the technique. Be- cause the aspect ratios of the particles in the deformed crystal depends solely on the applied mechanical strain, the method offers good control over particle aspect ratios, and can be used to create assemblies from different particle sizes. As long as the original template can be fabricated with long range order, the stretched anisotropic structures will also be ordered over large length scales. We demonstrate that it is also possible to simply align and stack 2D structures to create crystals in three dimensions. The assemble-and-stretch approach shows great promise for ultimately creating structures with complete photonic bandgaps. (Abstract shortened by UMI.).