Bubbles nucleating on superhydrophobic micropillar arrays under flow.

When a supersaturated aqueous solution flows over a microstructured, hydrophobic surface, bubbles tend to nucleate. Here, we control heterogeneous nucleation of gas bubbles from supersaturated CO2 solution. By designing the shape, size, and arrangement of hydrophobic micropillars and by adjusting the flow we obtain uniform nucleation patterns. It is possible to selectively turn nucleation on and off. We use laser scanning confocal microscopy to resolve nucleation in early stages at the micropillar-substrate intersection. Numerical simulations show a correlation between minute pressure drops behind micropillars and nucleation sites. Bubbles nucleate uniformly behind pillars of the same size. The flow profile further contributes to the uniform growth of the bubbles. We control heterogeneous nucleation by varying micropillar geometry or size, flow direction and rate. While nucleation behind square pillars is independent of the flow direction, nucleation behind round micropillars is coupled with the direction. Nucleation behind triangular micropillars is bifurcated. These observations pave the way for the replenishment of the gas layer entrapped in between hydrophobic surface features, needed for superhydrophobicity.

A study of photothermal laser ablation of various polymers on microsecond time scales.

To analyze the photothermal ablation of polymers, we designed a temperature measurement setup based on spectral pyrometry. The setup allows to acquire 2D temperature distributions with 1 µm size and 1 µs time resolution and therefore the determination of the center temperature of a laser heating process. Finite element simulations were used to verify and understand the heat conversion and heat flow in the process. With this setup, the photothermal ablation of polystyrene, poly(α-methylstyrene), a polyimide and a triazene polymer was investigated. The thermal stability, the glass transition temperature Tg and the viscosity above Tg were governing the ablation process. Thermal decomposition for the applied laser pulse of about 10 µs started at temperatures similar to the start of decomposition in thermogravimetry. Furthermore, for polystyrene and poly(α-methylstyrene), both with a Tg in the range between room and decomposition temperature, ablation already occurred at temperatures well below the decomposition temperature, only at 30-40 K above Tg. The mechanism was photomechanical, i.e. a stress due to the thermal expansion of the polymer was responsible for ablation. Low molecular weight polymers showed differences in photomechanical ablation, corresponding to their lower Tg and lower viscosity above the glass transition. However, the difference in ablated volume was only significant at higher temperatures in the temperature regime for thermal decomposition at quasi-equilibrium time scales.

Protein diffusion across the interface in aqueous two-phase systems.

We present a detailed study of the diffusive transport of proteins across a fluid phase boundary within aqueous two-phase systems. The aim of the work is to investigate whether local effects at the phase boundary cause a retardation of the diffusive transport between the phases. Possible modifications of interfacial mass transfer could be due to protein adsorption at the phase boundary or local electric fields from electric double layers. Experiments with a microfluidic system have been performed in which protein diffusion (bovine serum albumin and ovalbumin) within a bilaminated configuration of two phases containing polyethylene glycol and dextran is analyzed. A one-dimensional model incorporating phase-specific diffusion constants and the difference in chemical potential between the phases has been formulated. A comparison of experimental and simulation data shows a good overall agreement and suggests that a potential local influence of the phase boundary on protein transport is insignificant for the systems under investigation.

Sinusoidal crossflow microfiltration device--experimental and computational flowfield analysis.

We present an analysis of the flowfield inside a novel crossflow microfiltration device. The filter performance relies on shear focusing by means of a corrugated channel. The flow and shear stress characteristics inside the filter are studied by means of both micro Particle Image Velocimetry (micro-PIV) measurements and Computational Fluid Dynamics (CFD) analysis. We show that an increase of the shear rate by 55-85% as compared to a straight channel geometry is achieved for crossflow velocities ranging from 0.05 m s(-1)-0.8 m s(-1)(Re 5-70). This substantial increase in the local wall shear may improve filter performance in terms of reduced clogging and cell cake formation as compared to conventional crossflow filtration devices. Our current investigation, along with the fact that the filter employs no complex, three dimensional geometrical patterns, advanced pumping schemes, nor has a need for costly assembly and sealing procedures, indicates that the sinusoidal crossflow microfiltration module may serve as a technically and economically feasible solution for integrated lab-on-a-chip devices. Furthermore, the presented approach of shear-focusing may be beneficial in other bio-chemical contexts, such as cell lysis and surface chemistry.

Integrated polymer chip for two-dimensional capillary gel electrophoresis.

Two-dimensional (2D) gel electrophoresis (GE) is one of the most powerful methods for nucleic acid and protein separation, but generally suffers from high laboratory efforts connected with high analysis costs. Therefore, we herein present the development of a miniaturized 2D capillary GE (CGE) device which allows for an efficient protein separation in analysis times of about 1.5 h. This integrated 2D-CGE chip comprises a first channel for isoelectric focussing (IEF), a second specially designed transfer channel, 300 parallel micro channels, each having a cross section of 50 microm x 50 microm, and buffer reservoirs. The present work discusses fabrication aspects, in particular the combination of different microfabrication technologies, experimental separation performances of isoelectric focussing (IEF) and CGE, and presents computer simulations and first experimental results of protein transfer from the first to the second dimension.