Hier finden Sie ältere Meldungen über die Forschung am Fachgebiet Nano- und Mikrofluidik.

March 24, 2022

Electrically-induced Faraday waves

It is well-known that wave patterns (so-called Faraday waves) emerge on a vibrated liquid film. Not only mechanical forces can induce such wave patterns, but also electric forces, for example when keeping the liquid in a parallel-plate capacitor to which an AC voltage is applied. Remarkably, electrically induced Faraday waves have hardly been studied experimentally, although electric forcing offers options to control the wave patterns not available with mechanical forcing. We have studied the wave patterns of the electrically induced Faraday instability for the first time and compared them to theoretical predictions accounting for viscosity effects, with the result that there is a good agreement between experiments and theory.

S. Dehe, M. Hartmann, A. Bandopadhyay, and S. Hardt, The spatial structure of electrostatically forced Faraday waves, Journal of Fluid Mechanics 939 (2022), A6.

March 10, 2022

New group member: Salar Farrokhi

A few days ago, Salar Farrokhi joined our group. He will pursue his PhD and study dynamic wetting on microstructured surfaces.

Welcome Salar!

January 12, 2022

Coalescence control using electric fields

Controlling the coalescence of droplets is a key requirement in microfluidics. In that context, “controlling” often means avoiding the undesired coalescence of droplets. For this purpose, usually surfactants are employed, which, however, are detrimental in a number of applications. We have shown how droplet coalescence can be suppressed without surfactants simply by placing the droplets in a homogeneous electric field. The electric field redistributes the charges in such a way that a repulsive force between two nearby droplets is created. We believe that this principle may find widespread applications in droplet microfluidics wherever surfactants need to be avoided.

J. Hartmann, M. T. Schür, and S. Hardt, Manipulation and control of droplets on surfaces in a homogeneous electric field, Nature Communications 13 (2022), 289

January 5, 2022

From Taylor cones to surface dimples

It is well known that fluid interfaces disintegrate under sufficiently strong electric fields, leading to electrohydrodynamic tip streaming, often in form of a Taylor cone. However, when we studied the influence of a local electric field on an oil-water interface, an alternative deformation mode emerged: Here, the interface is pushed away from the electrode, and additional cone structures form at the rim of the dimple. Using experimental and numerical methods, we show that small droplets inside the oil phase play a crucial role in the dimple formation. They induce a background flow, which in turn perturbs the interface.

Sebastian Dehe and Steffen Hardt, Deformation modes of an oil-water interface under a local electric field: From Taylor cones to surface dimples, Phys. Rev. Fluids 6 (2021), 123702.

October 19, 2021

Partial droplet coalescence – where does it end?

When an electric field derives a water droplet to an interface between oil and water, under certain circumstances partial coalescence is observed. This means that instead of merging with the water volume, the droplet bounces back, whereupon it looses a part of its volume. The product droplet can again be driven to the oil-water interface, and the whole process can be repeated, resulting in sucessively smaller droplets. We have performed corresponding experiments with a large number of successive partial-coalescence events and could produce droplets with diameters of about 400 nm. The fact that we could not observe any smaller droplets is probably due to the optical resolution limits of our microscope. Would this process still work for droplet diameters of a few nanometers?

M. Shojaeian and S. Hardt, Manipulation of single sub-femtolitre droplets via partial coalescence in a direct-current electric field, Flow 1 (2021), E12.

October 18,2021

New group member: Arman Sadeghi

A few days ago, Professor Arman Sadeghi joined our group. Before that, he worked as Associate Professor at the University of Kurdistan, Iran. He is funded by the Alexander von Humboldt Foundation in terms of a Humboldt Research Fellowship.

Welcome Arman!

October 10, 2021

New group member: Qingwen Dai

A few days ago, Dr. Qingwen Dai joined our group. Before that, he worked as a lecturer at Nanjing University of Aeronautics & Astronautics, China. He is funded by the Alexander von Humboldt Foundation in terms of a Humboldt Research Fellowship.

Welcome Qingwen!

September 30,2021

Article published in Annual Review of Fluid Mechanics

Recently, an article entitled “Flow and Drop Transport Along Liquid-Infused Surfaces”, authored by Steffen Hardt and Glen McHale, was published in the Annual Review of Fluid Mechanics. In this article, recent developments related to liquid-infused surfaces are reviewed from a fluid mechanics perspective. It covers a spectrum ranging from single-phase flow along liquid-infused surfaces to dynamic wetting processes. A preprint of the article can be found here:

September 21, 2021

New group member: Satarupa Dutta

A few days ago, Dr. Satarupa Dutta joined our group. She recently obtained her PhD at the Indian Institute of Technology Guwahati and will study thermoelectric energy conversion in nanochannels in the framework of the EU project TRANSLATE, together with Rajkumar Sarma.

Welcome Satarupa!

August 24, 2021

Hydrodynamic dispersion in Hele-Shaw flows

In the 1950s, Taylor and Aris published their landmark results on the spreading of a dissolved species in unidirectional flow, i.e., in a tube or a channel, referred to as Taylor-Aris dispersion. We have considered flows between parallel-plates (so-called Hele-Shaw flows), which are no longer unidirectional, but may exhibit complex patterns owing to inhomogeneous wall boundary conditions. For such scenarios, we have asked how a dissolved species may spread. We have been able to reduce the 3D advection-diffusion equation for the concentration field to a 2D dispersion equation that may be viewed as a generalization of Taylor-Aris dispersion. We believe that in the future this equation could find widespread use in simplifying simulations of hydrodynamic dispersion in microfluidics.

S. Dehe, I. S. Rehm, and S. Hardt, Hydrodynamic dispersion in Hele-Shaw flows with inhomogeneous wall boundary conditions, Journal of Fluid Mechanics 925 (2021), A11.

August 6, 2021

New group member: Rajkumar Sarma

A few days ago, Dr. Rajkumar Sarma joined our group. He recently obtained his PhD at the Indian Institute of Technology Guwahati and will study thermoelectric energy conversion in nanochannels in the framework of the EU project TRANSLATE.

Welcome Rajkumar!

June 22, 2021

Start of EU project TRANSLATE

Recently, the EU project TRANSLATE, funded under the FET-Open scheme of the EU, was launched. Heavily relying on results we published a few years ago (, this project aims at establishing thermoelectric energy converters based on electrolyte-filled nanochannels. This could pave the way to the widespread use of inexpensive energy conversion devices made from highly abundant materials. Apart from our group, partners from the University College Cork (Ireland), the University of Latvia and the Spanish company Cidete form the TRANSLATE team. Further information can be found here:

May 7, 2021

Cloaking and shielding objects in a fluid flow

Among the best-known technologies deployed by Star Trek’s starships are their invisibility-inducing cloaking devices and their shields. Together with co-operation partners from the Technion, Israel, and IBM Research Europe we have developed a cloaking/shielding device for objects in microfluidic channels/chambers instead of starships. In cloaking mode, an object leaves the fluid flow around it undisturbed, and in shielding mode, the hydrodynamic forces on an object are eliminated. This principle may find a number of applications, for example in the manipulation of soft objects such as cells. See also

E. Boyko, V. Bacheva, M. Eigenbrod, F. Paratore, A. D. Gat, S. Hardt, and M. Bercovici, Microscale hydrodynamic cloaking and shielding via electro-osmosis, Physical Review Letters 126 (2021), 184502.

April 22, 2021

Breakup of a liquid bridge on a solid surface

Liquid bridges that become unstable and break up are found in numerous situations, for example when a droplet pinches off from a liquid reservoir. We have studied the breakup dynamics of water bridges wetting a hydrophobic surface using experiments and simulations. The dynamics is governed by a balance of inertial and capillary forces. We find that the liquid bridge decays into droplets in very much the same way as a free liquid bridge.

M. Hartmann, M. Fricke, L. Weimar, D. Gründing, T. Marić, D. Bothe, and S. Hardt, Breakup dynamics of capillary bridges on hydrophobic stripes, International Journal of Multiphase Flow 140 (2021),103582.

April 16, 2021

Protein separation at liquid-liquid interfaces

The separation of proteins according to specific properties such as size plays an eminent role in many processes of the biotech industry. We have recently demonstrated a new separation process for proteins in a microfluidic device. First, a protein mixture is electrophoretically transported towards a liquid-liquid interface. The interface represents a transport resistance to the proteins, such that some species adsorb at the interface more easily than others. Protein separation can be accomplished if one species crosses the interface, while the other gets adsorbed, which was demonstrated in the paper referred to below. We believe that in the future this new method of protein separation could extend the spectrum of industrial separation processes.

F. Gebhard, J. Hartmann and S. Hardt, Interaction of proteins with phase boundaries in aqueous two-phase systems under electric fields, Soft Matter 17 (2021), 3929-3936.

February 26, 2021

Stability of liquid rings in capillary tubes

Imagine introducing a small droplet into a capillary tube with circular cross section. “Small” means that the droplet volume is of the order of the tube diameter cubed or smaller. Such a droplet can exist in several shapes. It can form a liquid plug, a liquid ring or a sessile droplet at the tube wall. Interestingly, whether or not a liquid ring can exist depends on its volume and the contact angle of the liquid at the wall. We have explored the stability limits of liquid rings using analytical and numerical methods and represented the results in a stability map.

C. Lv and S. Hardt, Wetting of a liquid annulus in a capillary tube, Soft Matter 17 (2021), 1756-1772.

December 17, 2020

Electroosmotic flow (EOF) induced by surface acoustic waves

Surface acoustic waves (SAW) are widely used in microfluidics for transporting and mixing liquids. SAWs are acoustic waves travelling along the interface between a solid and a fluid. Up to now, the electrokinetic effects due to SAWs have remained largely unexplored. We have theoretically studied the EOF induced by SAWs and found that in small-scale channels, the EOF velocity may significantly exceed the velocity due to the acoustic field. Therefore, SAW-induced EOF may be useful as a method for pumping liquids through small scale channels.

M. Dietzel and S. Hardt, Electroosmotic flow in small-scale channels induced by surface-acoustic waves, Physical Review Fluids 5 (2020), 123702.

November 19, 2020

Fluid interfaces can become solid-like

As a stream encounters an obstacle, floating debris sometimes piles up in front of the obstacle, forming a dense layer on the water surface. At a microscopic scale, surfactants, molecules attached to the interface between two fluids, can suffer the same fate, immobilizing the interface upstream of an obstacle. ‘Super-hydrophobic’ surfaces with embedded gas pockets have been suggested for reducing drag on objects moving through water. With this in mind, we investigated the impact that surfactants have on this mechanism and found that parts of the gas-water interface become ‘solidified’ by surfactant pile-up.

T. Baier and S. Hardt, Influence of insoluble surfactants on shear flow over a surface in Cassie state at large Péclet numbers, Journal of Fluid Mechanics 907 (2020), A3.

October 2, 2020

Also fluid interfaces can be viscous

Usually, viscosity is a quantity associated with the bulk of a fluid. If, however, small particles or surfactants are adsorbed to a gas-liquid or liquid-liquid interface, the interface itself becomes viscous. In that case, the viscosity coefficients represent the dissipation due to the presence of particles or surfactants in an average manner. We have recently derived analytical expressions for the viscosity coefficients of a particle-laden fluid interface in the limit of low particle concentrations. These results could significantly simplify simulations of two-phase flows with particle-laden interfaces.

M. Eigenbrod and S. Hardt, The effective shear and dilatational viscosities of a particle-laden interface in the dilute limit, Journal of Fluid Mechanics 903 (2020), A26.

May 29, 2020

Intermediate wetting states on superhydrophobic surfaces

Together with cooperation partners from the Technion/Israel, we have studied the wetting states of hierarchical superhydrophobic surfaces, consisting of an array of micropillars that is decorated with nanoparticles. We find a multitude of intermediate states between the classical Cassie and Wenzel states. The transition from the Cassie state to these intermediate states is partially reversible. A summary of the main results can be found here .

B. Rofman, S. Dehe, V. Frumkin, S. Hardt, and M. Bercovici, Intermediate states of wetting on hierarchical superhydrophobic surfaces, Langmuir 36 (2020), 5517−5523.

May 29, 2020

Enhancement of electroosmotic flow on superhydrophobic surfaces

Together with cooperation partners from the Technion/Israel, we have studied the electroosmotic flow along superhydrophobic surfaces, augmented by gate electrodes. Via the charges created at the gas-liquid interfaces, the flow velocity can be increased by more than a factor of 10 compared to unstructured surfaces. In addition, the flow is entirely pH-independent. A summary of the main results can be found here .

S. Dehe, B. Rofman, M. Bercovici, and S. Hardt, Electro-osmotic flow enhancement over superhydrophobic surfaces, Physical Review Fluids 5 (2020), 053701.

May 6, 2020

Gas separation in a Knudsen pump

Knudsen pumps rely on gas-flows induced by temperature differences. We have studied the transport of gas mixtures along a Knudsen pump and observed differences in flow of the individual components of the mixture. Therefore, such a pump may be used for gas separation. This kind of separation of gas species by mass or size plays an important role for many applications in chemical engineering.

Baier, T., Hardt, S. Gas separation in a Knudsen pump inspired by a Crookes radiometer. Microfluid Nanofluid 24, 41 (2020).

March 2, 2020

“Tears of wine” in capillary tubes

“Tears of wine” is a phenomenon that can be observed in wine glasses: Driven by Marangoni stresses, a liquid film crawls up along the glass wall. Now imaging shrinking the diameter of the wine glass to one millimeter. In that case, ring-like structures are formed from the film. These liquid rings finally collapse and form plugs that are propelled along the capillary tube by evaporation. We have studied this phenomenon based on experiments and theory.

Reference: C. Lv, S. N. Varanakkottu, and S. Hardt, Liquid plug formation from heated binary mixtures in capillary tubes, Journal of Fluid Mechanics 889 (2020), A15. DOI: 10.1017/jfm.2020.80

February 19, 2020

Pattern formation in layers of DNA molecules

DNA molecules can be concentrated by electrophoretic accumulation at an interface between two immiscible polymer solutions. Apart from its relevance in applications, this process goes along with the formation of characteristic DNA concentration patterns, visible in the figure at the left. We have experimentally studied the concentration patterns and formulated a theory describing the pattern formation. The theoretical predictions based on linear stability analysis compare favorably with the experimental results.

Reference: S. Hardt, J. Hartmann, S. Zhao, and A. Bandopadhyay, Electric-field-induced pattern formation in layers of DNA molecules at the interface between two immiscible liquids, Physical Review Letters 124 (2020), 064501. DOI: 10.1103/PhysRevLett.124.064501

January 10, 2020

Species transfer using small droplets as carriers

Transferring chemical species or nanoparticles in a controlled way becomes challenging when very small species amounts are considered. We have found a solution to this problem, where a droplet with a diameter of the order of 5 µm serves as carrier for the cargo. The method is sketched in the schematic on the left. The droplet reciprocates between two aqueous reservoirs under the influence of a DC electric field. Upon touching the reservoir, the droplet reverses its charge and its direction of motion. We have studied the transfer of chemical species between the two reservoirs mediated by the droplet.

Reference: M. Shojaeian and S. Hardt, Mass transfer via femtoliter droplets in ping-pong mode, Physical Review Applied 13 (2020), 014015. DOI: 10.1103/PhysRevApplied.13.014015

December 12, 2019

Nanoparticle-wall interactions in gases

A sphere moving in the vicinity of a wall experiences an increased fluid-dynamic drag force compared to motion far from solid boundaries, influencing the adsorption of particles at surfaces. For a small particle inside a gas, the fluid dynamics of such problems becomes very involved, since the Navier-Stokes equation is no longer applicable. We have computed the corresponding drag force on a particle and find that it is much lower than the one predicted by the Navier-Stokes equation.

Reference: P. Goswami, T. Baier, S. Tiwari, C. Lv, S. Hardt, and A. Klar, Drag force on spherical particle moving near a plane wall in highly rarefied gas. Journal of Fluid Mechanics 883, 47 (2020). DOI:10.1017/jfm.2019.921. Link:

June 7, 2019

Non-contact electrostatic manipulation of droplets on liquid-infused surfaces

Liquid-infused surfaces have unique liquid-repelling properties. In comparison with conventional superhydrophobic surfaces, liquid-infused surfaces provide stable wetting states and pronounced self-healing properties. We have established the manipulation of highly mobile droplets sitting on silicone-oil infused surfaces by exploiting a non-uniform electric field between a grounded substrate and a non-touching pin electrode placed above it. Droplets are attracted towards the pin electrode, and translational velocities can exceed 1 cm/s.

Reference: N. Sinn, M. T. Schür, and S. Hardt, No-contact electrostatic manipulation of droplets on liquid-infused surfaces: Experiments and numerical simulations, Applied Physics Letters 114 (2019), 213704. DOI: 10.1063/1.5091836

May 8, 2019

Evaporation of droplets on surfaces with wettability patterns

On a surface with hydrophilic and hydrophobic stripes, an evaporating droplet breaks up if the wettability contrast between the different regions is high enough. A liquid bridge forms on the hydrophobic stripe, and when the width of the bridge reaches a critical value, it becomes unstable and breaks up rapidly. We have studied this process based on experiments, numerical simulations, and a heuristic analytical model.

Reference: M. Hartmann and S. Hardt, Stability of evaporating droplets on chemically patterned surfaces, Langmuir 35 (2019) 4868−4875. DOI: 10.1021/acs.langmuir.9b00172

March 13, 2019

Review article on surface-tethered polymers in confinement

In a number of microfluidic applications, polymers are tethered to the microchannel walls. Even in situations where the channel diameter is significantly larger than the radius of gyration of the polymer, confinement effects can become important. Our recent review article gives an overview of the research results in this area.

Reference: T. Roy, K. Szuttor, J. Smiatek, C. Holm, and S. Hardt, Conformation and dynamics of long-chain end-tethered polymers in microchannels, Polymers 11 (2019), 488. DOI: 10.3390/polym11030488

March 6, 2019

On-demand production of femtoliter droplets in microchannels

We have developed a method to produce monodisperse droplets inside microchannels with volumes in the femtoliter range on demand. The method utilizes pulsed electric fields deforming the interface between an aqueous and an oil phase and pinching off droplets. It can be applied even to solutions with a zero-shear rate viscosity more than ten thousand-fold higher than that of water. The droplets may serve as biological reaction compartments.

Reference: M. Shojaeian, F. X. Lehr, H. U. Göringer, and S. Hardt, On-demand production of femtoliter drops in microchannels and their use as biological reaction compartments, Analytical Chemistry 91 (2019) 3484-3491. DOI: 10.1021/acs.analchem.8b05063

November 22, 2018

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. We have developed a method enabling the trapping and manipulation of nano- and microparticles based on interfacial flows controlled by visible light. The inherent advantages of this method are the linear scaling of the trapping force with the particle diameter and the fact that the force is less dependent on particle properties than in the case of conventional methods.

Reference: C. Lv, S. N. Varanakkottu, T. Baier, and S. Hardt, Controlling the trajectories of nano/micro particles using light-actuated Marangoni flow, Nano Letters 18 (2018), 6924−6930. DOI: 10.1021/acs.nanolett.8b02814

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 the 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.

Reference: C. Lv, M. Eigenbrod, and S. Hardt, Stability and collapse of holes in liquid layers, Journal of Fluid Mechanics 855 (2018), 1130-1155. DOI:10.1017/jfm.2018.680

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.

Reference: M. Shojaeian and S. Hardt, Fast electric control of the droplet size in a microfluidic T-junction droplet generator, Applied Physics Letters 112 (2018) 194102. DOI: 10.1063/1.5025874

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.

Reference: A. Bandopadhyay and S. Hardt, Stability of horizontal viscous fluid layers in a vertical arbitrary time periodic electric field, Physics of Fluids 29 (2017) 124101. DOI: 10.1063/1.4999429

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;

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.

Reference: I. R. Damian, S. Hardt and C. Balan, From flow focusing to vortex formation in crossing microchannels, Microfluidics and Nanofluidics 21, 142 (2017). DOI: 10.1007/s10404-017-1975-7

April 10, 2017

Fulbright Fellow Professor Prashanta Dutta

Prof. Prashanta Dutta ( 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;

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.

September 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 04, 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 03, 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.

Reference: M. Dietzel and S. Hardt, Thermoelectricity in confined liquid electrolytes, Physical Review Letters 116, 225901 (2016)

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.

Reference: A. Dörr, S. Hardt, H. Masoud and H. A. Stone, Drag and diffusion coefficients of a spherical particle attached to a fluid interface, Journal of Fluid Mechanics 790 (2016), 607–618.

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.

Reference: A. Dörr and S. Hardt, Driven particles at fluid interfaces acting as capillary dipoles, J. Fluid Mech. 770 (2015), 5-26.