Colloidal Monolayers in Periodic Laser Fields

Colloids are small particles ranging from a few tens of nanometers to several micrometers. Suspensions of such colloids share many properties of molecular liquids and serve as paradigmatic model systems in condensed matter physics. The motion of the particles can be conveniently monitored by light microscopy, in contrast to molecular liquids, where neutron scattering or X-ray scattering is needed to resolve the dynamics in space and time. Colloids thus serve as big atoms whose interactions can be engineered to study the material properties of liquids and solids. In particular, they have been used extensively to gain insight into the glass transition the phenomenon that liquids become solids upon rapid cooling or compression without crystalizing. Despite significant efforts, a theoretical understanding of all facets of the glass problem is still missing. The goal of this project is to gain further insight into the glass transition phenomenon by exposing such colloidal liquids to extreme conditions. We take advantage of the fact that colloids can rather easily be manipulated by laser light as has been pioneered by Arthur Ashkin, whose experiments on optical tweezers have been awarded with the Nobel prize in 2018. More precisely, we employ a two- dimensional set-up, in other words a monolayer where the colloids can move only in a plane. Then we use two interfering laser beams to generate a light field displaying a sinusoidal modulation in intensity. The colloids are attracted to regions of high intensity, yet the strong repulsive interaction between the particles prevents the system to occupy only the high-intensity regions. Our setup allows manipulating the system by changing the density of the colloidal monolayer, the wavelength of the periodic modulation, as well as the intensity of the laser light. In our laboratory experiments, we will measure structural indicators, such as the pair-distribution function essentially the probability to find a particle at a given distance from a selected particle. Further quantities of interest are the diffusion coefficient, and the so-called intermediate scattering function characterizing spatio-temporal transport. We compare our experimental results to a novel theoretical approach extending previous successful theories by the periodic modulation via the laser light. We also perform computer simulations where trajectories of individual particles are generated and compare both to our theoretical and experimental results.

Summary: Structure and Dynamics of Colloidal Monolayers in Periodic Laser Fields Colloids are small particles ranging from a few tens of nanometers to several micrometers. Suspensions of such colloids share many properties of molecular liquids and serve as paradigmatic model systems in condensed matter physics. The motion of the particles can be conveniently monitored by light microscopy, in contrast to molecular liquids, where neutron scattering or X-ray scattering is needed to resolve the dynamics in space and time. Colloids thus serve as "big atoms" whose interactions can be engineered to study the material properties of liquids and solids. In particular, they have been used extensively to gain insight into the glass transition - the phenomenon that liquids become solids upon rapid cooling or compression without crystalizing. Despite significant efforts, a theoretical understanding of all facets of the glass problem is still missing. The goal of this project was to gain further insight into the glass transition phenomenon by exposing such colloidal liquids to extreme conditions. We take advantage of the fact that colloids can rather easily be manipulated by laser light as has been pioneered by Arthur Ashkin, whose experiments on "optical tweezers" have been awarded with the Nobel prize in 2018. More precisely, we employ a two-dimensional set-up, in other words a monolayer where the colloids can move only in a plane. Then we use two interfering laser beams to generate a light field displaying a sinusoidal modulation in intensity. The colloids are attracted to regions of high intensity, yet the strong repulsive interaction between the particles prevents the system to occupy only the high-intensity regions. Our setup allows manipulating the system by changing the density of the colloidal monolayer, the wavelength of the periodic modulation, as well as the intensity of the laser light. In our laboratory experiments, we have measured structural indicators, such as the pair-distribution function - essentially the probability to find a particle at a given distance from a selected particle. Further quantities of interest are the diffusion coefficient, and the so-called intermediate scattering function characterizing spatio-temporal transport. We have compared our experimental results to a novel theoretical approach extending previous successful theories by the periodic modulation via the laser light. We also have performed computer simulations where trajectories of individual particles are generated and compared both to our theoretical and experimental results.