We describe the motion of heated particles in a simple liquid, for which we can theoretically derive generalized fluctuation-dissipation relations that hold far from equilibrium, as we demonstrate both experimentally and via molecular-dynamics simulations. Due to persistent laser-light absorption, these particles excite a radially symmetric or asymmetric (Janus particles) temperature profile in the solvent, which affects their random (Brownian) and systematic (self-phoretic) motion. In case of a radially symmetric temperature profile, we show that the particles perform “hot Brownian motion” (HBM), with different effective temperatures pertaining to their various degrees of freedom. We moreover predict and experimentally observe a peculiar dependence of their diffusivity on the particle size. In case of an asymmetric temperature profile, we find a superimposed self-phoretic directed motion. To adjust the importance of this “active” motion relative to the random hot Brownian motion, the shape of the particle is modified by binding DNA molecules and DNA origami to Janus beads. The persistence of the directed transport can thereby greatly be enhanced.

In living systems DNA is subjected to considerable confinement but the molecule acts itself also as a confinement mechanism for cellular structures. Here we present investigations that study DNA under the confinement of supercoiling and within nanofluidic channels. Furthermore, we use DNA to confine the motion of microscopic and nanoscopic objects. In particular, we show how the motion dynamics of DNA-attached magnetic particles under external tension is affected and how DNA can confine the diffusion of enzymes to one dimension to follow the DNA contour.

Mass transfer processes in which specific interactions with environments lead to complex diffusion patterns, such as the occurrence of transient sub-diffusive behaviors or of heterogeneous diffusion, were studied by means of two different experimental techniques, namely single-particle tracking operating with single molecules and nuclear magnetic resonance operating with large molecular ensembles. As an important point, the combined application of these techniques allowed for a deeper insight into the microscopic diffusion mechanism in such complex systems, including those with broken ergodicity. Particle tracking concentrated on the “Influence of substrate surface properties on heterogeneous diffusion of probe molecules in ultrathin liquid films”. The mobility of liquids at solid-liquid interfaces is influenced by substrate heterogeneities. Here we study the distribution of surface silanols on differently treated silicon wafers with thermal oxide by confocal florescence microscopy of adsorbed Rhodamine G molecules. We further investigate the influence of the substrate properties on probe molecule diffusion in ultrathin liquid TEHOS films by single molecule tracking. The results are compared to simulations of two-layer diffusion employing heterogeneous substrates. Nuclear magnetic resonance has been applied to study translational diffusion of small organic molecules in nanopores and of polymer globules in the presence of larger polymer species. In both cases, the experiments revealed the occurrence of normal diffusion on the time scale of NMR experiments from ten to hundreds of milliseconds. While single particle tracking revealed the identical diffusivities for the former case, thus experimentally confirming the validity of the ergodicity theorem for diffusion, the discrepancies were noted for the latter case. More complex behavior revealing non-ergodic behavior for propagation of solid-liquid interfaces in disordered nanopores has further been studied using nuclear magnetic resonance cryoporometry. A common basis for comparing and analyzing the experimental observables accessed by the two methods is the distribution of diffusivities, which provides the probability of observing a given diffusivity fluctuation along a trajectory or in an ensemble. An overview of its properties is given and the advantages in analyzing heterogeneous, anisotropic, or anomalous diffusion processes are elaborated.

In this article, we review our results on diffusion and phase separation in lipid membranes, as well as on interaction of membranes with colloidal particles, biomacromolecules, and supramolecular assemblies, which were obtained within the framework of the Saxon Research Unit FOR 877 “From Local Constraints to Macroscopic Transport”.

A central part of soft matter physics is the investigation of effects in an active environment. These systems are driven out of equilibrium by a constant energy consumption. In biological systems, for instance, energy is consumed in the dynamic polymerization process of cytoskeletal filaments or by motor-filament interactions. These active processes convert chemical energy into mechanical work and impede a trapping of cellular structures in thermodynamically frozen states. Thus, active soft matter is crucial for biological systems to fulfill a broad range of tasks. Inherent physical principles relying on entropy maximizing arguments, however, cannot be easily switched off even in active systems. Cells might even employ these principles to accomplish certain tasks without the need to arrange elaborate, energy dissipating structures. Within the presented studies we demonstrate possibilities how biological relevant forces can be generated in the absence of any active accessory proteins. The presented studies are based on the cytoskeletal key components actin and microtubules. We demonstrate different approaches ranging from light induced softening to cross-linker expansion, which realize entropy driven contractions of the according system.

Experiments are described on the electrophoretic mobility of a single isolated colloid and the electro-osmotic response of the surrounding medium. For that optical tweezers are employed which enable one to trap a particle without any mechanical contact and to measure its position and the forces acting on it with high resolution (±2 nm, ±200 fN). In a custom-made microfluidic cell, the two effects are separated using the identical colloid. The electrophoretic response is found to be ~ 5 times stronger than the electroosmotic effect. It is phase-shifted with respect to the external electric field, hence giving rise to a complex electrophoretic mobility which can be theoretically described by a strongly damped driven harmonic oscillator model. The measured electrophoretic mobility in monovalent salt is found to be in agreement with computations combining primitive model molecular dynamics simulations of the ionic double layer with the standard electrokinetic model. Mobility reversal of a single colloid is observed for trivalent ionic solutions (LaCl3) at ionic strengths > 10−2 mol/l. The latter is in quantitative agreement with a numerical model in which ion specific attractive forces are taken into consideration.

Using a combination of analytical theory and newly developed numerical algorithms, we analyze the most pertinent conformational characteristics of three paradigmatic types of polymers in disordered environments: (i) flexible polymers in quenched, self-similar disorder as represented by a self-avoiding random walk on a critical percolation cluster, (ii) semiflexible polymers in quenched, steric disorder as represented by an equilibrium hard-disk fluid and (iii) semiflexible polymers subject to the random energy landscape that emerges from a surrounding network of similar semiflexible polymers.

No description available.

No description available.

No description available.