Microfluidic free-flow electrophoresis for proteomics-on-a-chip

A new free-flow electrophoresis microchip with integrated permeable membranes was developed, and different substances were separated by free-flow zone electrophoresis, free-flow isoelectric focusing and free-flow field step electrophoresis. This chip contained a new type of membranes enabling a stable carrier flow with a perpendicular electrical current. Due to this chip configuration, the device performance and efficiency were superior to recenty published alternative systems in terms of separation resolution and sample capacity. The results furthermore indicate that even better results are possible. Analytes were separated and focused within hundreds of milliseconds whereby only nanoliters of samples were consumed. In addition, a new sample steering method was demonstrated during free-flow zone electrophoresis, allowing the specific sorting of various components. As an alternative, a free-flow electrophoresis chip was developed with integated platinum electrodes, whereby the generation of gas bubbles caused by electrolysis was successfully suppressed by chemical means. Gas bubbles generated by electrolysis are major concern in free-flow electrophoresis systems in general leading to distorted separation. Based on the results, a fourth free-flow chip was developed with an integrated surface plasmon resonance gold detection region. Although fabrication was successful, certain hurdles, in particular surface chemistry issues still remain to be overcome to perform separation and real-time detection of biological samples within this hyphenated micro device. A strategy for proteomics-on-a-chip was developed aiming at the separation of antigens that play a role in autoimmune diseases. In addition two new continuous flow microfluidic chips were developed allowing for continuous biochemical reactions of surface patterning applications. These devices could be of further interest in future, in particular in more complex analytical systems related to proteomics-on-a-chip

Modeling sublimation by computer simulation: morphology dependent effective energies

Solid-On-Solid (SOS) computer simulations are employed to investigate the sublimation of surfaces. We distinguish three sublimation regimes: layer-by-layer sublimation, free step flow and hindered step flow. The sublimation regime is selected by the morphology i.e. the terrace width. To each regime corresponds another effective energy. We propose a systematic way to derive microscopic parameters from effective energies and apply this microscopical analysis to the layer-by-layer and the free step flow regime. We adopt analytical calculations from Pimpinelli and Villain and apply them to our model. Key-Words: Computer simulations; Models of surface kinetics; Evaporation and Sublimation; Growth; Surface Diffusion; Surface structure, morphology, roughness, and topography; Cadmium tellurideComment: 12 pages, 6 Postscript figures, uses psfig.st

Application of turbulence modeling to predict surface heat transfer in stagnation flow region of circular cylinder

A theoretical analysis and numerical calculations for the turbulent flow field and for the effect of free-stream turbulence on the surface heat transfer rate of a stagnation flow are presented. The emphasis is on the modeling of turbulence and its augmentation of surface heat transfer rate. The flow field considered is the region near the forward stagnation point of a circular cylinder in a uniform turbulent mean flow. The free stream is steady and incompressible with a Reynolds number of the order of 10 to the 5th power and turbulence intensity of less than 5 percent. For this analysis, the flow field is divided into three regions: (1) a uniform free-stream region where the turbulence is homogeneous and isotropic; (2) an external viscid flow region where the turbulence is distorted by the variation of the mean flow velocity; and, (3) an anisotropic turbulent boundary layer region over the cylinder surface. The turbulence modeling techniques used are the kappa-epsilon two-equation model in the external flow region and the time-averaged turbulence transport equation in the boundary layer region. The turbulence double correlations, the mean velocity, and the mean temperature within the boundary layer are solved numerically from the transport equations. The surface heat transfer rate is calculated as functions of the free-stream turbulence longitudinal microlength scale, the turbulence intensity, and the Reynolds number

Computing stationary free-surface shapes in microfluidics

A finite-element algorithm for computing free-surface flows driven by arbitrary body forces is presented. The algorithm is primarily designed for the microfluidic parameter range where (i) the Reynolds number is small and (ii) force-driven pressure and flow fields compete with the surface tension for the shape of a stationary free surface. The free surface shape is represented by the boundaries of finite elements that move according to the stress applied by the adjacent fluid. Additionally, the surface tends to minimize its free energy and by that adapts its curvature to balance the normal stress at the surface. The numerical approach consists of the iteration of two alternating steps: The solution of a fluidic problem in a prescribed domain with slip boundary conditions at the free surface and a consecutive update of the domain driven by the previously determined pressure and velocity fields. ...Comment: Revised versio

A two-dimensional model of low-Reynolds number swimming beneath a free surface

Biological organisms swimming at low Reynolds number are often influenced by the presence of rigid boundaries and soft interfaces. In this paper we present an analysis of locomotion near a free surface with surface tension. Using a simplified two-dimensional singularity model, and combining a complex variable approach with conformal mapping techniques, we demonstrate that the deformation of a free surface can be harnessed to produce steady locomotion parallel to the interface. The crucial physical ingredient lies in the nonlinear hydrodynamic coupling between the disturbance flow created by the swimmer and the free boundary problem at the fluid surface