Echo state networks for modeling turbulent convection.

Turbulent Rayleigh-Bénard convection (RBC) is one of the very prominent examples of chaos in fluid dynamics with significant relevance in nature. Meanwhile, Echo State Networks (ESN) are among the most fundamental machine learning algorithms suited for modeling sequential data. The current study conducts reduced order modeling of experimental RBC.

From rectangular to diamond shape: on the three-dimensional and size-dependent transformation of patterns formed by single particles trapped in microfluidic acoustic tweezers.

Generally, the pattern formed by individual particles trapped inside a microfluidic chamber by a two-dimensional standing acoustic wave field has been considered only the result of the acoustic radiation force. Previous studies showed that particles can be trapped at the local minima and maxima of the first-order pressure and velocity fields. Thus, either a rectangular or a diamond pattern can be formed solely depending on the particle size, when the acoustic field is unchanged, and the material properties of the particles and the fluid are fixed.

Highly efficient passive Tesla valves for microfluidic applications.

A multistage optimization method is developed yielding Tesla valves that are efficient even at low flow rates, characteristic, e.g., for almost all microfluidic systems, where passive valves have intrinsic advantages over active ones.

Three-dimensional heating and patterning dynamics of particles in microscale acoustic tweezers.

Acoustic tweezers facilitate a noninvasive, contactless, and label-free method for the precise manipulation of micro objects, including biological cells. Although cells are exposed to mechanical and thermal stress, acoustic tweezers are usually considered as biocompatible. Here, we present a holistic experimental approach to reveal the correlation between acoustic fields, acoustophoretic motion and heating effects of particles induced by an acoustic tweezer setup.

On the acoustically induced fluid flow in particle separation systems employing standing surface acoustic waves - Part II.

Particle separation using surface acoustic waves (SAWs) has been a focus of ongoing research for several years, leading to promising technologies based on Lab-on-a-Chip devices. In many of them, scattering effects of acoustic waves on suspended particles are utilized to manipulate their motion by means of the acoustic radiation force (). Due to viscous damping of radiated waves within a fluid, known as the acoustic streaming effect, a superimposed fluid flow is generated, which additionally affects the trajectories of the particles by drag forces.

On the acoustically induced fluid flow in particle separation systems employing standing surface acoustic waves - Part I.

By integrating surface acoustic waves (SAW) into microfluidic devices, microparticle systems can be fractionated precisely in flexible and easily scalable Lab-on-a-Chip platforms. The widely adopted driving mechanism behind this principle is the acoustic radiation force, which depends on the size and acoustic properties of the suspended particles. Superimposed fluid motion caused by the acoustic streaming effect can further manipulate particle trajectories and might have a negative influence on the fractionation result.

Combining magnetic forces for contactless manipulation of fluids in microelectrode-microfluidic systems.

A novel method to drive and manipulate fluid in a contactless way in a microelectrode-microfluidic system is demonstrated by combining the Lorentz and magnetic field gradient forces. The method is based on the redox-reaction [Fe(CN)]/[Fe(CN)] performed in a magnetic field oriented perpendicular to the ionic current that crosses the gap between two arrays of oppositely polarized microelectrodes, generating a magnetohydrodynamic flow. Additionally, a movable magnetized CoFe micro-strip is placed at different positions beneath the gap.

Marangoni convection at electrogenerated hydrogen bubbles.

Electrolytic gas evolution is a fundamental phenomenon occurring in a large number of industrial applications. In these processes gas bubbles are formed at the electrode from a supersaturated solution. Since dissolved gases can change the surface tension, a gas concentration gradient may cause the surface tension to vary locally at the interface of the gas bubble.

3D measurement and simulation of surface acoustic wave driven fluid motion: a comparison.

The characterisation of the fluid motion induced by the acoustic streaming effect is of paramount interest for novel microfluidic devices based on surface acoustic waves (SAWs), e.g. for a detailed description of the achievable mixing efficiency and thus the design of such devices.

Simultaneous three-dimensional temperature and velocity field measurements using astigmatic imaging of non-encapsulated thermo-liquid crystal (TLC) particles.

A combination of cutting edge developments is presented to characterize three-dimensional (3D) temperature and velocity fields in microscopic flows. An emulsion of non-encapsulated thermo-liquid crystal (TLC) micro spheres, with a narrow size distribution is used to track the flow's motion and temperature distribution. A state-of-the-art light engine, which combines the spectrum of six light pipes, provides a balanced illumination which allows for strong and detectable color patterns across the TLC's temperature response range.

Logarithmic boundary layers in strong Taylor-Couette turbulence.

We provide direct measurements of the boundary layer properties in highly turbulent Taylor-Couette flow up to Re=2×106) (Ta=6.2×10(12)) using high-resolution particle image velocimetry and particle tracking velocimetry. We find that the mean azimuthal velocity profile at the inner and outer cylinder can be fitted by the von Kármán log law u+=1/κ lny+ +B.

Parallax correction for precise near-wall flow investigations using particle imaging.

For the basic understanding of turbulence generation in wall-bounded flows, precise measurements of the mean velocity profile and the mean velocity fluctuations very close to the wall are essential. Therefore, three techniques are established for high-resolution velocity profile measurements close to solid surfaces: (1) the nanoprobe sensor developed at Princeton University, which is a miniaturization of a classical hot-wire probe [Exp. Fluids 51, 1521 (2011)]; (2) the laser Doppler velocimetry (LDV) profile sensor, which allows measurement of the location of the particles inside the probe volume using a superposition of two fringe systems [Exp.

Clarifying the mechanism of reverse structuring during electrodeposition in magnetic gradient fields.

Deviating from the common expectation, magnetoelectrochemical structuring during deposition of diamagnetic ions was demonstrated, very recently. To achieve this, electrochemically inert paramagnetic ions have to be added to the electrolyte and the deposition has to be performed in a magnetic gradient field. A reverse structuring occurs, yielding thinner deposits near high gradient regions.

In situ analysis of three-dimensional electrolyte convection evolving during the electrodeposition of copper in magnetic gradient fields.

A novel three-dimensional particle tracking velocimetry technique was used to examine the flow during electrodeposition of Cu. For the first time electrode-normal, circumferential, and radial velocities were spatially resolved during deposition in superimposed low and high magnetic gradient fields. In this way the complex interaction of magnetic field gradient force and Lorentz force induced convective effects could be measured and analyzed.