Tailoring of phononic band structures in colloidal crystals

We report an experimental study of the elastic properties of a two-dimensional (2D) colloidal crystal subjected to light-induced substrate potentials. In agreement with recent theoretical predictions [H. H. von Grünberg and J. Baumgartl, Phys. Rev. E 75, 051406 (2007).10.1103/PhysRevE.75.051406] the phonon band structure of such systems can be tuned depending on the symmetry and depth of the substrate potential. Calculations with binary crystals suggest that phononic band engineering can be also performed by variations of the pair potential and thus opens novel perspectives for the fabrication of phononic crystals with band gaps tunable by external fields.     

Depletion potentials induced by charged colloidal rods

We present direct depletion potential measurements for a single colloidal sphere close to a wall in suspensions of charged colloidal rods. In contrast to earlier studies of purely entropic systems (Helden et al. Phys. Rev. Lett. 2003, 90, 048301), here electrostatic interactions are important. These enhance the depletion attraction and lead to repulsive parts in the interaction potentials, indicating correlation effects between the rods.     

Experimental verification of an exact evanescent light scattering model for TIRM

Total internal reflection microscopy (TIRM) is a method for the precise measurement of interaction potentials between a spherical colloidal particle and a wall. The method is based on single-particle evanescent wave light scattering. The well-established model used to interpret TIRM data is based on an exponential relation between scattering intensity and particle wall distance. However, applying this model for a certain range of experimental parameters leads to significant distortions of the measured potentials. Using a TIRM setup based on a two-wavelength illumination technique, we were able to directly measure the intensity distance relation revealing deviations from an exponential decay. The intensity-distance relations could be compared to scattering simulations taking into account exact experimental parameters and multiple reflections between a particle and the wall. Converging simulation results were independently obtained by the T-matrix method and the discrete sources method (DSM) and show excellent agreement with experiments. Using the new scattering model for data evaluation, we could reconstruct the correct potential shape for distorted interaction potentials as we demonstrate. The comparison of simulations to experiment intrinsically yields a new method to determine absolute particle-wall distances, a highly desired quantity in TIRM experiments.     

Experimental observation of structural crossover in binary mixtures of colloidal hard spheres

Using confocal microscopy, we investigate the structure of binary mixtures of colloidal hard spheres with size ratio q=0.61. As a function of the packing fraction of the two particle species, we observe a marked change of the dominant wavelength in the pair-correlation function. This behavior is in excellent agreement with a recently predicted structural crossover in such mixtures. In addition, the repercussions of structural crossover on the real-space structure of a binary fluid are analyzed. We suggest a relation between crossover and the lateral extension of networks containing only equally-sized particles that are connected by nearest-neighbor bonds. This is supported by Monte Carlo simulations which are performed at different packing fractions and size ratios.     

Peptide-related alterations of membrane-associated water: deuterium solid-state NMR investigations of phosphatidylcholine membranes at different hydration levels

Deuterated water associated with oriented POPC bilayers was investigated before and after the addition of 2 mol% peptide. Membranes in the presences of antimicrobial-(LAH4), pore-forming- (the segments M2 of influenza A and S4 of the domain I of rat brain sodium channels) or lysine-containing model peptides (LAK1 and LAK3) were investigated by (2)H and proton-decoupled (31)P solid-state NMR. The NMR spectra were recorded as a function of hydration in the range between 15 and 93% relative humidity and of sample composition. In the presence of peptides an increased association of water is observed. A quantitative analysis suggests that the peptide-induced changes in the lipid bilayer packing have a significant effect on membrane-water association. The quadrupolar splittings of (2)H(2)O at a given degree of hydration indicate that the changes of the water deuterium order parameter are specific for the peptide sequence and the lipid composition.     

Bilayer sample for fast or slow magic angle oriented sample spinning solid-state NMR spectroscopy

An alternative setup for Magic Angle Oriented Spinning Spectroscopy is proposed. Samples were prepared by orienting lipid bilayers onto polymer films, which were wrapped into a spiral so as to fit into 4 or 7 mm MAS rotors. This geometry resulted in narrow line widths and a higher upper spinning limit when compared to the conventional MAOSS setup with stacked glass plates. Whereas orientational information was extracted from low spinning spectra, fast spinning will be applicable to high-resolution multidimensional NMR pulse sequences.     

The interaction of helical polypeptides with biological model membranes

Proton-decoupled solid-state ¹⁵N NMR spectroscopy was used to investigate helical peptides reconstituted into oriented phospholipid bilayers. Hydrophobic channel peptides such as the N-terminal region of Vpu of human immunodeficiency virus (HIV-1) adopt transmembrane orientations, whereas amphipathic peptide antibiotics are oriented parallel to the bilayer surface. The alignment of helical peptides in lipid membranes was analysed in some detail using model peptides. In particular, peptides with pH-dependent topology and a series of peptides that allow one to study the contributions of specific interactions were designed. The energies of transfer of several amino acids from the in-plane to transmembrane localisation were determined. In addition, the alignment of peptides and phospholipids under conditions of hydrophobic mismatch have been investigated in considerable detail.     

Direct measurement of three-body interactions amongst charged colloids

Three-body interactions amongst three charged colloidal particles are measured in a deionized aqueous solution. Two of the particles are confined to an optical line trap while the third one is approached by means of a focused laser beam. From the observed particle configurations we extract the three-body potential which is found to be attractive and roughly of the same magnitude and range as the pair interactions. In addition, numerical calculations are performed which show qualitative agreement with the experimental results.     

Archimedean-like colloidal tilings on substrates with decagonal and tetradecagonal symmetry

Two-dimensional colloidal suspensions subjected to laser interference patterns with decagonal symmetry can form an Archimedean-like tiling phase where rows of squares and triangles order aperiodically along one direction (J. Mikhael et al., Nature 454, 501 (2008)). In experiments as well as in Monte Carlo and Brownian dynamics simulations, we identify a similar phase when the laser field possesses tetradecagonal symmetry. We characterize the structure of both Archimedean-like tilings in detail and point out how the tilings differ from each other. Furthermore, we also estimate specific particle densities where the Archimedean-like tiling phases occur. Finally, using Brownian dynamics simulations we demonstrate how phasonic distortions of the decagonal laser field influence the Archimedean-like tiling. In particular, the domain size of the tiling can be enlarged by phasonic drifts and constant gradients in the phasonic displacement. We demonstrate that the latter occurs when the interfering laser beams are not ideally adjusted.