Optical tweezers in concentrated colloidal dispersions : Manipulating and imaging individual particles

Abstract

Using a laser beam that is focused down to a diffraction-limited spot, particles with a size ranging from several nanometers up to tens of micrometers can be trapped and manipulated. This technique, known as "optical tweezers" or "optical trapping", has been used in a wide variety of (interdisciplinary) fields and its use is expanding fast. Recent improvements allow for new exciting and promising applications.

First an introduction to the technique of optical tweezers is given and the setup that combines optical tweezers with confocal microscopy is described. By time-sharing a single laser beam, hundreds of optical traps can be created and changed dynamically. Independent three-dimensional trapping and imaging of individual particles is possible because two microscope objectives are used, one below and one above the sample.

We demonstrate that large two-dimensional patterns of colloidal particles can be created using optical tweezers by positioning particles on a coated surface. These structures of colloidal particles can subsequently be used as masks for colloidal lithography. These masks can be modified further using ion beam deformation or, alternatively, chemical methods. Large arrays of metal nanoparticles with sizes in the range of tens of nanometers have been created using these techniques.

Using our setup we have also studied crystallization and melting in colloidal dispersions. A single optical trap was used on collections of particles to crystallize two- and three-dimensional colloidal dispersions. With specially synthesized mixtures of core-shell colloidal particles it is now possible to manipulate individual articles selectively inside concentrated dispersions. Arrays of optical tweezers were used to trap core-shell particles that act as nuclei for homogeneous and heterogeneous nucleation in concentrated colloidal dispersions. We show a single particle driven through a three-dimensional colloidal crystal.