October 22, 2018
Electrophoresis of surface particles
The electrophoresis of particles immersed in a liquid is a well-studied phenomenon. However, what happens when a particle attached to a liquid surface translates along that surface driven by an electric field has been largely unknown. We have computed the electrophoretic mobility of a particle at the interface between two fluids with large viscosity contrast. For thin Debye layers, the Smoluchowki mobility is recovered. Generally, the mobility depends on the contact angle between the fluids and the particle. We have also calculated the interfacial deformation caused by Debye layer around the particle.
Reference: M. Eigenbrod, F. Bihler, and S. Hardt, Electrokinetics of a particle attached to a fluid interface: Electrophoretic mobility and interfacial deformation, Physical Review Fluids 3 (2018), 103701. DOI: 10.1103/PhysRevFluids.3.103701
October 8, 2018
Relaxation of surface-tethered polymers under moderate confinement
When long-chain polymers attached to a surface become stretched and the stretching force is released, they relax back to their coiled state within a characteristic time, denoted the relaxation time. We have shown that the polymers “feel” the presence of a second wall even if the distance between the two walls h is significantly larger than the radius of gyration of the polymers. As a result, the relaxation time increases. These results are relevant for microchannel flows where the channel walls are decorated with polymers.
Reference: J. Hartmann, T. Roy, K. Szuttor, J. Smiatek, C. Holm, and Steffen Hardt, Relaxation of surface-tethered polymers under moderate confinement, Soft Matter 14 (2018), 7926-7933. DOI: 10.1039/c8sm01246f
October 1, 2018
Stability of holes in liquid films
If you spill liquid over a surface, it forms a film with a thickness of the order of its capillary length. If the surface is bounded and you did not spill enough liquid, a hole will form. The stability of the hole depends on its size. There is a threshold size below which it collapses. For a water film, the collapse is dominated by inertia. We studied the stability and collapse of holes in liquid films in detail and compared the experimental results with mathematical models and simulations.
September 21, 2018
Thermophoresis of Janus particles at large Knudsen numbers
Thermophoresis, the motion of a particle along a thermal gradient, is exploited for deposition of aerosols on cooled surfaces. For non-symmetric particles it may be desirable to induce deposition with a preferred orientation of the particle. As a model system for this situation, we consider a spherical Janus particle having dissimilar reflective properties for gas molecules on its opposite hemispheres and investigate the interplay between rotational diffusion and thermophoretic motion on the orientation of the particle.
Reference: Tobias Baier, Sudarshan Tiwari, Samir Shrestha, Axel Klar, and Steffen Hardt, Thermophoresis of Janus particles at large Knudsen numbers, Phys. Rev. Fluids 3, 094202 (2018), DOI: 10.1103/PhysRevFluids.3.094202
August 15, 2018
Instabilities in microchannel gas-liquid flow
In a parallel-plates microchannel where the walls are coated with liquid films, separated by a gas layer in the middle, the gas flow triggers characteristic instabilities of the films. Interestingly, the instability patterns become synchronized between the two films, i.e. the upper film “knows” what the lower film does. For example, in many cases the mirror-symmetric mode (with respect to x2, see the figure) dominates. We have performed a detailed analysis of the coupled instability modes.
Reference: M. Vécsei, M. Dietzel, and S. Hardt, Interfacial instability of liquid films coating the walls of a parallel-plate channel and sheared by a gas flow, Microfluidics and Nanofluidics 22 (2018), 91. DOI: 10.1007/s1040 4-018-2111-z
May 14, 2018
Individual droplet size control in a microfluidic T-junction droplet generator
Electric fields can be used to control the size of individual aqueous droplets produced at a microfluidic T-junction. Under a sufficiently strong electric field, the droplet diameter is about half of the diameter obtained without electric field. Using electric field pulses with a duration of the order of the inverse droplet production frequency, the size control of individual droplets produced in a continuous stream becomes possible. Based on that, arbitrary droplet size sequences can be encoded.
January 31, 2018
New Tutorial “CFD-based simulation and optimization of microfluidic components”
This new tutorial will equip students with the basic methodology needed to design and optimize microfluidic components on the computer. The course language will be English.
December 11, 2017
Excitation of fluid interfaces with AC electric fields
An interface between a dielectric and a conducting fluid can be excited using an AC electric field. When the electric field strength exceeds a threshold voltage, patterns with a characteristic wavelength appear at the fluid interface. Different from the Faraday instability, a related case where the interface becomes unstable due to a periodic acceleration, the response of the system is very complex when excited by a superposition of different frequencies. We have analyzed the instability modes using Floquet theory. Corresponding instabilities may be utilized for pattern formation based on self-organization.
August 17, 2017
Stretching of confined surface-tethered polymers in pressure-driven flow
Knowledge on the stretching and conformations of linear long-chain polymers in micro-confinement is of prime importance for applications such as functional surfaces with grafted polymer brushes or sequencing of DNA molecules. We experimentally characterized the stretching and conformational changes of surface-anchored long-chain double-stranded DNA molecules between parallel surfaces under pressure-driven flow. One main result is that the fractional extension of the molecules is a unique function of the product of the wall shear stress and the molecular contour length, with a weak influence of confinement. The experimental results are corroborated by a simple scaling analysis and coarse-grained lattice-Boltzmann / molecular dynamics simulations:
Reference: T. Roy, K. Szuttor, J. Smiatek, C. Holm and S. Hardt, Stretching of surface-tethered polymers in pressure-driven flow under confinement, Soft Matter (2017); doi: 10.1039/C7SM00306D; http://pubs.rsc.org/en/content/articlehtml/2017/sm/c7sm00306d
August 15, 2017
Some surprises in flow focusing
Flow focusing is a widely applied scheme in microfluidics, employed for creating a single-file stream of particles/cells or for creating small droplets in a two-phase flow. When the flow rate of the central stream is reduced to very small values, flow patterns very different from the common flow-focusing scenario emerge. In that case, instead of one single focused stream, two or four streams are created. The corresponding flow patterns are illustrated in the figure on the left. In the future, such flow-focusing patterns may be exploited for inertial particle sorting, among others.
April 10, 2017
Fulbright Fellow Professor Prashanta Dutta
Prof. Prashanta Dutta (http://www.mme.wsu.edu/people/faculty/faculty.html?duttaP) has come to the TU Darmstadt with a Fulbright grant from the United States (US) government to collaborate with researchers at the Institute for Nano- and Microfluidics on electric field driven transport in micro- and nanodevices. The Fulbright program is the flagship international educational exchange initiative sponsored by the United State government and the primary goal of his visit is to develop a long-term collaborative relationship between US and German researchers in the micro/nano/biofluidics area.
January 27, 2017
Knudsen pump inducing gas flow normal to temperature gradient
Knudsen pumps transport gas by exploiting temperature variations imposed by the channel boundaries, offering the advantage of not containing any moving parts. We analyzed a Knudsen pump with a temperature field generated by an applied temperature difference between the channel walls, where the ratchet-shaped wall is equipped with varying reflection properties on different sections. The figure shows streamlines and velocity magnitudes within a channel bounded by triangular teeth which are half-specularly and half-diffusely reflecting. The use of specularly reflecting patches massively increases the mass flux compared to diffusely reflecting walls.
Reference: V. Shahabi, T. Baier, E. Roohi and S. Hardt, Thermally induced gas flows in ratchet channels with diffuse and specular boundaries, Scientific Reports 7, 41412 (2017); doi:10.1038/srep41412; www.nature.com/articles/srep41412
January 02, 2017
Humboldt Fellow Dr. Aditya Bandopadhyay
On Jan 2, 2017, Dr. Aditya Bandopadhyay has joined our group as a Humboldt postdoctoral fellow. His research interests are electrokinetics, electrohydrodynamics and reactive mixing. Prior to his arrival in Germany, he has completed his education from IIT Kharagpur, India and has undertaken postdoctoral research in Geosciences Rennes, France. We look forward to a productive research collaboration.
Sept 15, 2016
Humboldt Fellow Professor Dominik Barz
On Sept 1, 2016, Professor Dominik Barz of Queen’s University/Canada joined our group. He is supported by the Humboldt Foundation and pursues a research program in the field of electrokinetic flows. We are looking forward to a fruitful co-operation.
July 4, 2016
GIF Research Grant
We have won a research grant by the German Israeli Foundation (GIF). Together with Dr. Moran Bercovici from the Israel Institute of Technology we will study methods for tailoring and reconfiguring complex electroosmotic flow patterns in a channel-free microfluidic device.
June 3, 2016
Thermoelectricity in confined liquid electrolytes
The electric field induced in a bulk phase of a liquid electrolyte exposed to a temperature gradient is attributed to different thermophoretic mobilities of the dissolved ion species. We have shown that such Soret-type ion thermodiffusion is not required to induce thermoelectricity even in the simplest electrolyte if it is confined between walls carrying a charge density. The space charge of the electric double layer leads to selective ion diffusion driven by a temperature-dependent electrophoretic ion mobility, which —for narrow channels— may cause thermovoltages larger in magnitude than for the classical Soret effect. On the left, the corresponding (scaled) Seebeck coefficient is plotted for different values of the surface charge density against the (scaled) channel width.
February 19, 2016
Stokes drag on a sphere translating along a fluid-fluid interface
Presumably the most well-known achievement of George Gabriel Stokes is his formula for the drag force experienced by a sphere translating in an unbounded fluid at low Reynolds numbers. However, in some scenarios small particles are attached to an interface between two immiscible fluids, constrained to tangential motion relative to the interface. We have analyzed this situation and derived a generalization of Stokes’ law for large viscosity contrast and small capillary number. The image at the left is a sketch of the corresponding streamlines.
October 26, 2015
Structuring thin liquid films utilizing the Bénard-Marangoni instability
If a system of two superposed liquid films is exposed to a temperature gradient normal to the films, the upper layer exhibits convection cells which deform the lower thin film in an identical pattern. This effect can be seen in the figure above, which displays the hexagonal convection cells and the humps of the thin film below. The principle could find applications in fabricating regular, highly ordered surface structures.
Reference: I.Nejati, M. Dietzel and S. Hardt, Conjugated liquid layers driven by the short-wavelength Bénard-Marangoni instability: experiment and numerical simulation, J. Fluid Mech., 783 (2015), 46-71
July 01, 2015
Institute for Nano- and Microfluidics participates in LOEWE Cluster CompuGene
Yesterday the government of the federal state of Hessen approved approx. 4.4 million Euros funding for the LOEWE Cluster CompuGene. CompuGene will focus on the development of artificial genetic circuits which may be employed for various purposes, for example for the production of certain rare substances. We will contribute by exploring microfluidic methods for studying genetic circuits in a scenario as close as possible to in vivo conditions.
April 07, 2015
Results on motion of a microsphere along a fluid interface published in Journal of Fluid Mechanics
A microsphere driven along an interface between two fluids of highly different viscosities experiences a drag force differing from the well-known Stokes drag. Additionally, the viscous flow around the moving particle deforms the fluid-fluid interface from its equilibrium shape while the particle assumes a tilted orientation. The figure below depicts a spherical particle with a pinned three-phase contact line moving from left to right with the lower fluid having higher viscosity than the upper. The corresponding deformation of the fluid interface results in a pair interaction between particles.