Scientific publications

Flow of a liquid inside a helical tube is composed of axial and circumferential components, the latter arising because of its specific geometry. For a gas-liquid two-phase flow inside a helical tube, the coupled effect of these two flow components leads to a variety of flow patterns, for example, slug, bubble, and stratified flow. We present here a novel triple-helical microchannel, in which, the two-phase flow is found to engender several additional flow patterns not observed with the conventional geometries, for example, the parallel and oscillating annular flow and even simultaneous occurrence of several such patterns. We show that the transition between these patterns depends not only on the fluid rates of the two liquids but also on the helix angle. We have presented detailed phase diagrams to elaborate these effects. We have examined also the effect of channel geometry on the specific features of these flow patterns.

In their comment (M. Lagos et al., Scripta Mater. (2012), http://dx.doi.org/10.1016/j.scriptamat.2012.04.018), Lagos et al. propose a two-dimensional plasticity model based on grain boundary sliding to explain the deformation behavior of ultrathin Cu and Ta/Cu film systems on polyimide substrates. Here, we critically discuss their comments and include new results obtained by peak profile analysis of the original in situ diffraction data; they strongly suggest that the deformation behavior of the different film systems is very likely not self-similar as claimed by Lagos et al.

We report direct measurement of surface deformation in soft solids due to their surface tension. Gel replicas of poly(dimethysiloxane) masters with rippled surfaces are found to have amplitudes that decrease with decreasing gel modulus. Surface undulations of a thin elastomeric film are attenuated when it is oxidized by brief exposure to oxygen plasma. Surface deformation in both cases is modeled successfully as driven by surface tension and resisted by elasticity. Our results show that surface tension of soft solids drives significant deformation, and that the latter can be used to determine the former.

Adhesion tests were performed on single macroscopic pillars as model systems for artificial gecko surfaces. Polydimethylsiloxane macropillars with 400 μm diameter and aspect ratios ranging from 1 to 5 were fabricated. The tip geometries were modified to achieve spherical, flat and mushroom shaped tips. Unlike spherical tips, flat tip pillars exhibited a strong angle dependency of the pull-off force. For mushroom shaped tips the pull-off force was tilt angle dependent only for low preload, where no complete tip contact was formed. No clear influence of pillar aspect ratio on adhesion could be identified. Implications of the results for the adhesion performance of fibrillar arrays are addressed.

Current adhesion measurement setups designed for experiments on bioinspired fibrillar surfaces, either commercial or constructed in-house, do not allow adhesion measurements with in situ visualization, high resolution, high force range, and controlled alignment at the same time. In this paper a new adhesion tester is presented, which enables contact experiments with controlled tilt angle (accuracy of ±0.02°). This allows the use of flat probes and thus greatly simplifies the determination of experimental parameters such as pull-off strength or Young's modulus. The deflection of a double-clamped glass beam is measured by laser interferometry with an accuracy of ±60 nm, which yields a precise force measurement over three orders of magnitude force range without changing the glass beam. Contact formation and detachment events can be visualized in situ. The current adhesion tester is designed for force measurements in the range of 1 µN to 1 N and fills the gap between macroscopic tests and atomic force microscopy measurements.

Purpose: A powerful principle in nature is the presence of surface patterns to improve specific characteristics or to enable completely new functions. Here, we present two case studies where bioinspired surface patterns based on the adhesive system of geckos may be applied for biomedical applications: residue-free adhesion to skin and gecko-inspired suture threads for knot-free wound closure. Methods: Gecko-inspired skin adhesives were fabricated by soft lithography of polydimethylsiloxane with successive inking and dipping steps. Their adhesion was measured using a home built adhesion tester designed for patterned surfaces. Preliminary lap shear tests on the back of a human hand were also performed. Commercial suture threads from different materials were patterned in the group of A. del Campo at the Max-Planck-Institute for Polymer Research (Mainz, Germany) using oxygen plasma. The treated threads were pulled through artificial skin in both directions measuring the peak force and the pull through force. Results and Conclusions: Unpatterned reference samples of the skin adhesive did not stick to human skin, while the patterned samples all showed notable adhesion up to 1.2 Newton for a sample size of approximately 3 cm². First results with the patterned suture threads indicated that the surface patterning of the thread has only a minor effect on the pull-through forces. To achieve knot-free sewing the surface geometry of the suture threads needs to be optimized and more realistic testing procedures, e.g. testing on human skin, are necessary.

While directional effects in adhesion and locomotion have in general been generated by creating symmetry breaking topographic features on the surface of a soft bodied object, here we present a novel method for imparting this effect to thin adhesive layers by embedding liquid filled microchannels arranged in pairs with specific intra and inter pair distances. The adhesive exhibits uniform adhesion in classical peel tests when both the channels are filled with either air or a wetting liquid. But the asymmetric effect shows up when only one of the channels in the pair is filled with the liquid. The liquid alters the surface tension of the inner wall of the channel, which results in bulging deformation of the thin skin of the adhesive over the channel. The bulging however remains asymmetric, the extent of asymmetry depending on the intra-pair spacing between the channels. Besides the bulging effect, filling in one channel of a pair with liquid also leads to an asymmetric variation in its modulus. As a result, when an adherent is peeled off the adhesive from two opposite directions, significantly different adhesion strengths result. A similar directional effect also results when channels of two different diameters are used in the pair, thus opening up the possibility of generating several different adhesion strengths simply by altering the geometric features of the embedded microstructure and its filling status. We show also that for both channels in a pair filled with liquid, the adhesion strength increases significantly, by over 60 times of what is achieved for a smooth, featureless, adhesive layer.

The abnormal growth of individual (1 0 0) oriented grains is monitored by the in situ electron backscatter diffraction technique for more than 24 h at three different annealing temperatures (90 °C, 104 °C and 118 °C) in 1-5 μm thick Cu films on polyimide substrates. The (1 0 0) grain growth velocity increases with higher film thickness and annealing temperature, as suggested by an earlier model by Thompson and Carel. As a result, the final (1 0 0) texture fraction becomes more dominant for higher annealing temperatures and larger film thicknesses. The Thompson-Carel model, however, predicts that the (1 1 1) grains will preferably grow at temperatures up to 118 °C. Our calculations of the driving forces revealed that in addition to minimization of the strain energy (due to the thermal mismatch between film and substrate) and of the surface energy, the energy stored in the dislocations plays a decisive role in grain growth. Our observations can be understood by the notion that initially available (1 0 0) grain nuclei start to grow very rapidly, due to dislocation annihilation, and thus "overrun" the (1 1 1) grains in size.

A hydrophobic starch derivative is used for safe and enhanced delivery of anticancer agents. The synthesis and characterization of propyl starch with a controlled degree of substitution to modulate the release of the encapsulated hydrophobic drug is reported. The application of this polymer for formulating nanoparticles of docetaxel, an anti-cancer agent effective against numerous types of cancers but possessing intrinsic formulation difficulties is described. The solvent emulsification/diffusion technique is used and the synthesis is optimized with respect to several formulation parameters. Uptake studies with these nanoparticles indicate their enhanced internalization by the cancerous cells and their peri-nuclear localization.