Controlling the Trajectories of Nano/Micro Particles Using Light-Actuated Marangoni Flow.

The ability to manipulate small objects and to produce patterns on the nano- and microscale is of great importance, both with respect to fundamentals and technological applications. The manipulation of particles with diameters of the order of 100 nm or below is a challenge because of their Brownian motion but also because of the scaling behavior of methods such as optical trapping. The unification of optical and hydrodynamic forces is a potential route toward the manipulation of tiny objects.

Thermally induced gas flows in ratchet channels with diffuse and specular boundaries.

A net gas flow can be induced in the gap between periodically structured surfaces held at fixed but different temperatures when the reflection symmetry along the channel axis is broken. Such a situation arises when one surface features a ratchet structure and can be augmented by altering the boundary conditions on different parts of this surface, with some regions reflecting specularly and others diffusely. In order to investigate the physical mechanisms inducing the flow in this configuration at various Knudsen numbers and geometric configurations, direct simulation Monte Carlo (DSMC) simulations are employed using transient adaptive subcells for collision partner selection.

Energy harvesting through gas dynamics in the free molecular flow regime between structured surfaces at different temperatures.

For a gas confined between surfaces held at different temperatures the velocity distribution shows a significant deviation from the Maxwell distribution when the mean free path of the molecules is comparable to or larger than the channel dimensions. If one of the surfaces is suitably structured, this nonequilibrium distribution can be exploited for momentum transfer in a tangential direction between the two surfaces. This opens up the possibility to extract work from the system which operates as a heat engine.

Increasing the sensitivity of microfluidics based immunoassays using isotachophoresis.

We have developed a microfluidic device that enhances the sensitivity of protein immunoassays by preconcentrating the protein sample using isotachophoresis (ITP). Two approaches were followed to study the sensitivity gain achieved that way. The first approach was using antibody-coated magnetic beads loaded into a microchannel to capture the proteins within the ITP sample zone.

Thermally driven flows between a Leidenfrost solid and a ratchet surface.

The significance of thermally driven flows for the propulsion of Leidenfrost solids on a ratchet surface is studied based on a numerical solution of the Boltzmann equation. The resulting flow patterns are dominated by vortices developing at the edges of the ratchet teeth. In a previous analysis it had been claimed that thermally driven flows could cause the propulsion of Leidenfrost objects.

Propulsion mechanisms for Leidenfrost solids on ratchets.

We propose a model for the propulsion of Leidenfrost solids on ratchets based on viscous drag due to the flow of evaporating vapor. The model assumes pressure-driven flow described by the Navier-Stokes equations and is mainly studied in lubrication approximation. A scaling expression is derived for the dependence of the propulsive force on geometric parameters of the ratchet surface and properties of the sublimating solid.

Transport and separation of micron sized particles at isotachophoretic transition zones.

Conventionally, isotachophoresis (ITP) is used for separation of ionic samples according to their electrophoretic mobilities. We demonstrate that the scope of ITP applications may be extended toward particle concentration and separation. Owing to the distributions of electrolyte concentration and electric field inside a transition zone between two electrolytes, a number of different forces act on a small particle.

Thermocapillary flow on superhydrophobic surfaces.

A liquid in Cassie-Baxter state above a structured superhydrophobic surface is ideally suited for surface driven transport due to its large free surface fraction in close contact to a solid. We investigate thermal Marangoni flow over a superhydrophobic array of fins oriented parallel or perpendicular to an applied temperature gradient. In the Stokes limit we derive an analytical expression for the bulk flow velocity above the surface and compare it with numerical solutions of the Navier-Stokes equation.

Micro contactor based on isotachophoretic sample transport.

It is demonstrated how isotachophoresis (ITP) in a microfluidic device may be utilized to bring two small sample volumes into contact in a well-controlled manner. The ITP contactor serves a similar purpose as micromixers that are designed to mix two species rapidly in a microfluidic channel. In contrast to many micromixers, the ITP contactor does not require complex channel architectures and allows a sample processing in the spirit of "digital microfluidics", i.