The objective of our work is to further develop optical techniques to manipulate and image nanometer sized entities. For example, by extending the capabilities of holographic optical trapping and fluorescence microscopy we wish to make and study novel all optical devices. By combining holographic imaging and microscopic shear cells we study the mechanical properties of biological proteins. In addition, we complement our work by synthesizing specialized colloids with properties tailored for our applications.
Faceting and growth of droplets on micro-fabricated surfaces
We look at the problem of morphology selection in faceting and roughening of quasicrystals in equilibrium, which is virtually uncharted territory. To this aim we use an analog system of liquid spreading on a textured landscape. Our aim in this ongoing study is to understand how the symmetry, scale and composition of the patterned substrate govern droplet spreading, shrinking, and translation. As part of this effort, we explore the influence of substrate symmetry on hydrophobicity, with potential applications for self-cleaning surfaces and anti-bacterial coatings.
Initially we studied droplet shape formation on the different tilings. We found that while on a square lattice and on a hexagonal lattice the droplet almost always adopts a symmetric shape, this is not the case for the Penrose tiling. On a square lattice, octagons were readily created, for Penrose tilings facets formed in two crystallographic directions: along the line connecting a pentagon center to its vortices and perpendicular to that line (Fig. 1), which is in contrast to the theoretical prediction.
Figure 1: Facet formation of a droplet spreading on a pillar array ordered in a 5-fold symmetric quasicrystal lattice, (a) parallel to the pentagon directions (surprisingly), and (b) perpendicular to the pentagon directions (as was predicted).
Hydrodynamic interactions between driven and confined colloidal particles
Hydrodynamic interaction between colloidal particles are usually of little importance in self assembly under quasi-equilibrium conditions, however, they could play an important role in assembling structures in a driven system. In order to improve our understanding of the effect hydrodynamic interactions have on directed self assembly we study a simple driven-dissipative system of colloids confined to move on a ring.
Surprisingly, we find that hydrodynamic interactions lead these systems to exhibit behaviors ranging from periodic motion to intermittent motion to weakly chaotic motion depending on temperature. Moreover, it seems that the diffusion of a single particle in the ensemble of rotating colloids is anomalous.
Recently, we found that the curvature of the path traveled by the particles breaks the symmetry of the inter-particle hydrodynamic interactions resulting in particle pairing. Stokesian dynamics simulations allowed us to study the low temperature behavior of the paired particles. The simulation revealed that at low temperature particle pairing leads to a well define set of microscopic configuration which the system can adopt. These low temperature microscopic states become unstable above a threshold temperature which leads to the previously seen intermittent behavior. We show that one can relate the average angular velocity to the configuration of our system (Fig. 2), and explicitly, the collective mobility of the colloidal system can be calculated analytically based on the pair mobility.
Figure 2: Averaged and normalized angular velocity of particles driven on a ring as a function of temperature for a two particle vortex and a four particle vortex. At low temperature several microscopic configuration can be observed depending on initial conditions.