Noise-enhanced performance of ratchet cellular automata

We present the first experimental realization of a ratchet cellular automaton (RCA) which has recently been suggested as an alternative approach for performing logical operations with interacting (quasi)particles. Our study was performed with interacting colloidal particles which serve as a model system for other dissipative systems, i.e., magnetic vortices on a superconductor or ions in dissipative optical arrays. We demonstrate that noise can enhance the efficiency of information transport in RCA and consequently enables their optimal operation at finite temperatures.     

Solid state NMR studies of oligourea foldamers: interaction of 15N-labelled amphiphilic helices with oriented lipid membranes

Synthetic oligomers that are derived from natural polypeptide sequences, albeit with unnatural building blocks, have attracted considerable interest in mimicking bioactive peptides and proteins. Many of those compounds adopt stable folds in aqueous environments that resemble protein structural elements. Here we have chemically prepared aliphatic oligoureas and labeled them at selected positions with (15)N for structural investigations using solid-state NMR spectroscopy. In the first step, the main tensor elements and the molecular alignment of the (15)N chemical shift tensor were analyzed. This was possible by using a two-dimensional heteronuclear chemical shift/dipolar coupling correlation experiment on a model compound that represents the chemical, and thereby also the chemical shift characteristics, of the urea bond. In the next step (15)N labeled versions of an amphipathic oligourea, that exert potent antimicrobial activities and that adopt stable helical structures in aqueous environments, were prepared. These compounds were reconstituted into oriented phospholipid bilayers and the (15)N chemical shift and (1)H-(15)N dipolar couplings of two labeled sites were determined by solid-state NMR spectroscopy. The data are indicative of an alignment of this helix parallel to the membrane surface in excellent agreement with the amphipathic character of the foldamer and consistent with previous models explaining the antimicrobial activities of α-peptides.     

Developing DNP/Solid-State NMR Spectroscopy of Oriented Membranes

Dynamic nuclear polarization (DNP)/solid-state nuclear magnetic resonance (NMR) spectroscopy bears great potential for the investigation of membrane-associated polypeptides which can often be produced only in small amounts and which need to be ‘diluted’ in lipid bilayer environments to adopt or maintain their functional structure. Here we present investigations using biradicals, such as TOTAPOL and bTbK, for solid-state NMR signal enhancement using DNP in the context of lipid membranes. By transferring polarization from electron to nuclear spins using microwave irradiation signal enhancement factors of up to 13 are obtained with TOTAPOL and up to 17 with bTbK. The possible reasons why these factors are below those obtained in glassy samples of bulk solvents (40–60 under similar conditions) are evaluated and discussed. In order to further ameliorate the enhancement factors the physico-chemical characteristics of TEMPOL, TOTAPOL, bTbK, and bCTbK, such as their partitioning between hydrophilic and hydrophobic solvents or their stability under different environmental conditions are presented. Finally, having provided proof-of-concept that DNP/solid-state NMR measurements can be performed with oriented membrane samples work in progress is presented on the development of a flat-coil probe for DNP/solid-state NMR experiments on oriented membranes.     

Low frequency acoustic and dielectric measurements on glasses

The acoustic and dielectric properties of different glasses at audio frequencies and temperatures below 1 K have been investigated with the vibrating reed and a capacitance bridge technique. We found the temperature dependence of the absorption of vitreous silica (Suprasil W) to agree with the predictions of the tunneling model which is commonly used to explain the low temperature behaviour of amorphous materials. The variation of the sound velocity and of the dielectric constant, however, shows significant deviations from the expected behaviour which cannot be accounted for by a simple modification of the model. Instead, it seems to be necessary to introduce a temperature dependence of some relevant model parameters. Moreover, at very low temperatures (T < 0.1 K) the sound velocity strongly depends on the excitation levels. The absence of this effect at higher temperatures proves that it can be ascribed to a nonlinear response of tunneling systems. Similar results were found in sound velocity measurements on a cover glass and on a superconducting metallic glass (Pd₃₀Zr₇₀, T_c = 2.6 K), which indicates that these features are a general aspect of the dynamics of tunneling states in glasses. In contrast to the insulating glasses we found that in Pd₃₀Zr₇₀ also the internal friction is strain dependent.     

Measurement of permeability of microfluidic porous media with finite-sized colloidal tracers

We present two methods how the permeability in porous microstructures can be experimentally obtained from particle tracking velocimetry of finite-sized colloidal particles suspended in a liquid. The first method employs additional unpatterned reference channels where the liquid flow can be calculated theoretically and a relationship between the velocity of the particles and the liquid is obtained. The second method takes advantage of a time-dependent pressure drop that leads to an exponential decrease in the particle velocity inside a porous structure. From the corresponding decay time, the permeability can be calculated independently of the particle size. Both methods lead to results comparable with permeabilities derived from numerical simulations.     

Phase behavior of colloidal molecular crystals on triangular light lattices

We investigate the melting process of a two-dimensional charge-stabilized colloidal system in the presence of a triangular substrate potential. This potential is formed by an optical interference pattern that allows the substrate strength to be varied continuously. By means of an additional scanned optical tweezer the particle density can be adjusted to different numbers m of colloidal particles per substrate minima; here we concentrate on the case of trimers, i.e., m = 3. Because trimers exhibit additional internal degrees of rotational freedom, the phase behavior of such a system is very different from homogeneous or one-dimensional periodic substrate potentials.     

Phase transitions of colloidal monolayers in periodic pinning arrays

We study the phase behavior of two-dimensional paramagnetic colloidal systems on square pinning arrays, the latter being created by a holographic optical tweezer technique. When the particle interaction strength is decreased, a transition from an incommensurate to a commensurate solid is observed. At even smaller pair potentials, the interstitial particles start to melt, whereas the particles at the substrate pinning sites are still localized. Our results are in good agreement with recent numerical studies on vortex melting in periodic pinning arrays.     

Single-file diffusion of colloids in one-dimensional channels

We study the diffusive behavior of colloidal particles which are confined to one-dimensional channels generated by scanning optical tweezers. At long times t, the mean-square displacement is found to scale as t(1/2), which is expected for systems where single-file diffusion occurs. In addition, we experimentally obtain the long-time, self-diffusive behavior from the short-time collective density fluctuations of the system as suggested by a recent analytical approach [Phys. Rev. Lett. 90, 180602 (2003)].     

Active Brownian motion tunable by light

Active Brownian particles are capable of taking up energy from their environment and converting it into directed motion; examples range from chemotactic cells and bacteria to artificial micro-swimmers. We have recently demonstrated that Janus particles, i.e.?gold-capped colloidal spheres, suspended in a critical binary liquid mixture perform active Brownian motion when illuminated by light. In this paper, we investigate in more detail their swimming mechanism, leading to active Brownian motion. We show that the illumination-borne heating induces a local asymmetric demixing of the binary mixture, generating a spatial chemical concentration gradient which is responsible for the particle's self-diffusiophoretic motion. We study this effect as a function of the functionalization of the gold cap, the particle size and the illumination intensity: the functionalization determines what component of the binary mixture is preferentially adsorbed at the cap and the swimming direction (towards or away from the cap); the particle size determines the rotational diffusion and, therefore, the random reorientation of the particle; and the intensity tunes the strength of the heating and, therefore, of the motion. Finally, we harness this dependence of the swimming strength on the illumination intensity to investigate the behavior of a micro-swimmer in a spatial light gradient, where its swimming properties are space-dependent.     

Shear thinning and local melting of colloidal crystals

Phenomena such as shear thinning and thickening, occurring when complex materials are exposed to external forces, are generally believed to be closely connected to changes in the microstructure. Here, we establish a direct and quantitative relation between shear thinning in a colloidal crystal and the surface area of the locally melted region by dragging a probe particle through the crystal using optical tweezing. We show that shear thinning originates from the nonlinear dependence of the locally melted surface area on the drag velocity. Our observations provide unprecedented quantitative evidence for the intimate relation between mechanical properties and underlying changes in microscopic structure.