Colloids in one dimensional random energy landscapes

Abstract

Individual colloidal particles have been studied experimentally in a one dimensional random potential with energies that follow a Gaussian distribution. This rough, noise-like potential has been realised using a holographic optical set-up, which allows the width of the distribution to be varied. For different widths, the particle trajectories were followed and the particle dynamics characterised by, for example, the mean square displacement, non-Gaussian parameter, van Hove function, time-dependent diffusion coefficient and residence time distribution. The values obtained for these observables are consistent with the static properties of the system, in particular the barrier height distribution, which was obtained by a detailed characterisation of the tweezer-like set-up. The dynamics display three distinct behaviours: at short times normal diffusion, subsequently an extended regime of localisation within the different minima of the potential and finally a very slow approach towards long-time diffusive behaviour, for which diffusion coefficients consistent with theoretical predictions have been found.

Delay-induced transport in a rocking ratchet under feedback control

“…Indeed, ratchet potentials acting on colloids can be easily realized by using laser beams [10,43,45] (optical line trap), and the position of a colloidal particle (or the mean position of many particles) is accessible, e.g., by video microscopy. Moreover, feedback control based on the particle position (or mean position) has already been realized experimentally, e.g.…”

Colloidal dynamics over a tilted periodic potential: Nonequilibrium steady-state distributions

“…Here an external potential field is used to mimic the effect of an energy landscape, which is usually imposed by the surrounding molecules to a test particle. Similar attempts have also been made in the study of colloidal transport and diffusion in a 1D optical trap (optical tweezers) with either a periodic or random variation of the laser light intensity [21][22][23][24]. Understanding the effect of the external force on thermally activated kinetics is a concern of a common class of transport problem, such as particle separation by electrophoresis [25,26], electromigration of atoms on the surface of metals [27] and semiconductors [28], motion of a three-phase contact line under the influence of an unbalanced capillary force [29], control of crystal growth [30], and design of nanoscale machineries [31,32].…”

“…While the laser-generated potential is a useful system for the study of colloidal dynamics over different potentials [21][22][23][24], the colloidal platform has several advantages in the experimental implementation. (i) It is a pure potential field and does not have any nonconservative component, as the laser beam does [50,51].…”

Speckle optical tweezers: micromanipulation with random light fields

“…Moreover, one major challenge common to all these techniques is the light scattering occurring in optically complex media, such as biological tissues, turbid liquids and rough surfaces, which naturally gives rise to apparently random light fields known as speckles [20]. Earlier experimental works showed trapping of atoms and particles in a gas by high-intensity speckle light fields [21][22][23][24], while both static and timevarying speckle fields were related to the emergence of anomalous diffusion in colloids [25][26][27][28][29]. Recently, we derived a theory to describe the motion of a Brownian particle in a speckle light field which allowed us to demonstrate numerically how a speckle field can be used to control the motion a Brownian particle in the limit of particles much smaller than the light wavelength (dipole approximation) [29].…”