Andreas Best

Effect of capillary pressure and surface tension on the deformation of elastic surfaces by sessile liquid microdrops: An experimental investigation

Sessile liquid drops are predicted to deform an elastic surface onto which they are placed because of the combined action of the liquid surface tension at the periphery of the drop and the capillary pressure inside the drop. Here, we show for the first time the in situ experimental confirmation of the effect of capillary pressure on this deformation. We demonstrate micrometer-scale deformations made possible by using a low Youngs modulus material as an elastic surface. The experimental profiles of the deformed surfaces fit well the theoretical predictions for surfaces with a Youngs modulus between 25 and 340 kPa.

Three-dimensional ferroelectric domain visualization by Cerenkov-type second harmonic generation.

We show that focusing a laser light onto the boundary between antiparallel ferroelectric domains leads to the non-collinear generation of two second harmonic (SH) beams. The beams are emitted in a plane normal to the domain boundaries at the angles that satisfy the Cerenkov-type phase matching condition. Moreover, these beam disappear when the laser light is focused on a homogenous part of a single domain. We utilize this effect for 3-dimensional visualization of fine details of the ferroelectric domain pattern with a submicron accuracy.

Direct measurements of hydrophobic slippage using double-focus fluorescence cross-correlation.

We report the results of direct measurements of velocity profiles in a microchannel with hydrophobic and hydrophilic walls, using a new high-precision method of double-focus spatial fluorescence cross correlation under a confocal microscope. In the vicinity of both walls the measured velocity profiles do not go to zero by supplying a plateau of constant velocity. This apparent slip is proven to be due to a Taylor dispersion, an augmentation by shear diffusion of nanotracers in the direction of flow. Comparing the velocity profiles near the hydrophobic and hydrophilic walls for various conditions shows that there is a true slip length due to hydrophobicity. This length, of the order of several tens of nanometers, is independent of the electrolyte concentration and shear rate.

Impact of micropatterned surfaces on neuronal polarity.

Experimental control over cellular polarity in a neuronal network is a promising tool to study synapse formation and network behavior. We aimed to exploit a mechanism described by Stenger et al. [J. Neurosci. Methods 82 (1998) 167] to manipulate the direction of axonal versus dendritic outgrowth on a micropattern. The group had used laser ablation to create patterns of aminated silanes for cell attachment on a background of repellent fluorinated silanes. The pattern offered continuous adhesive pathways for axonal and interrupted pathways for dendritic outgrowth. By microcontact printing, we created similar patterns containing continuous and interrupted pathways consisting of extracellular matrix proteins on a background of polystyrene. Neuronal polarity was determined on the functional level through double patch clamp measurements, detecting synapses and their orientation. Although our pattern reproduced the properties that were assumed to be critical for the described effect, namely contrasting pathways of different adhesiveness, we failed to reproduce the above results. It is indicated that other qualities of alternative pathways than mere differences in adhesiveness are required to orient neuronal polarity in vitro. We suggest that the effect observed by Stenger et al. has to be attributed to less universal characteristics of the micropattern, e.g. to the specific chemical groups that were utilized.

Diffusion in Polymer Solutions Studied by Fluorescence Correlation Spectroscopy

We employed fluorescence correlation spectroscopy (FCS) to study the diffusion of molecular and macromolecular tracers in polystyrene solutions over a broad range of concentrations (c) and molecular weights (M(w,m)) of the matrix polymer. Molecular tracer diffusion scales only with the matrix concentration and superimposes on a single, nonpolymer specific, curve. On the contrary, the diffusion of macromolecular tracers in solutions of matrix polymers with M(w,m) sufficiently larger than the tracer molecular weight scales with c/c(p)*, where c(p)* is the tracer overlap concentration. We further demonstrate that FCS can address local and global dynamics simultaneously.

Dielectric and mechanical relaxation of cresolphthalein- dimethylether

From pressure–volume–temperature measurements, in combination with published dielectric spectroscopy results, we assess the volume dependence of structural relaxation in cresolphthalein–dimethylether (KDE). Structural relaxation in KDE cannot be described as either a volume activated or thermally activated process, and unlike results for some other molecular glass formers, temperature is not the dominant control variable. The inflection point of the PVT data yields a glass temperature, Tg, that corresponds to a dielectric relaxation time equal to 10 s. There is a marked variation with pressure, dTg/dP=307u200aK/GPa at low pressure. We also carried out dynamic mechanical measurements on KDE near Tg. The shape of the mechanical and dielectric relaxation functions, as well as both the magnitude and temperature dependence of the respective relaxation times, are the same. Such equivalence is not generally expected, and arises herein due to the rigid structure of the KDE molecule.

Arrays of microlenses with variable focal lengths fabricated by restructuring polymer surfaces with an ink-jet device

We report of a method for fabricating two-dimensional, regular arrays of polymer microlenses with focal lengths variable between 0.2 and 4.5 mm. We first make concave microlenses by ink-jetting solvent on a polymer substrate with a commercial drop-on-demand device. Solvent evaporation restructures the surface by a series of combined effects, which are discussed. In the second step we obtain convex elastomeric microlenses by casting the template made in the first step. We demonstrate the good optical quality of the microlenses by characterising their surfaces with atomic force microscopy and white light interferometry, and by directly measuring their focal lengths with ad-hoc confocal laser scanning microscopy.

Nonlinear prism coupling of waveguides of the conjugated polymer MEH-PPV and their figures of merit for all-optical switching

We used intensity-dependent prism coupling of poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) waveguides to measure the nonlinear absorption coefficients and nonlinear refractive indices of these waveguides in the range 700–1200 nm. The material’s strong nonlinearity combined with low linear waveguide losses ( 1150 nm.

Nanocarrier for Oral Peptide Delivery Produced by Polyelectrolyte Complexation in Nanoconfinement

The hydrophilic peptide YY (PYY) is a promising hormone-based antiobesity drug. We present a new concept for the delivery of PYY from pH-responsive chitosan-based nanocarriers. To overcome the drawbacks while retaining the merits of the polyelectrolyte complex (PEC) method, we propose a one-pot approach for the encapsulation of a hydrophilic peptide drug in cross-linked PEC nanocarriers. First, the hydrophilic peptide is encapsulated via polyelectrolyte complexation within water-in-oil miniemulsion droplets. In a second step, the PEC surface is reinforced by controlled interfacial cross-linking. PYY is efficiently encapsulated and released upon pH change. Such nanocarriers are promising candidates for the fight against obesity and, in general, for the oral delivery of protein drugs.

Monitoring the Dynamics of Phase Separation in a Polymer Blend by Confocal Imaging and Fluorescence Correlation Spectroscopy

The phase separation of the polymer blend polystyrene/poly(methyl phenyl siloxane) (PS/PMPS) is studied in situ by laser scanning confocal microscopy (LSCM) and by fluorescence correlation spectroscopy (FCS) at macroscopic and microscopic length scales, respectively. It is shown for the first time that FCS when combined with LSCM can provide independent information on the local concentration within the phase-separated domains as well as the interfacial width.