Institute for Nano- and Microfluidics

We are concerned with transport phenomena in fluids on the nano- and micrometer scale. In that context we are especially interested in studying fundamentals, with the intention to pave the way for novel applications.

Our research extends over a broad thematic spectrum and combines experimental, theoretical and numerical approaches. Nanoscale gas kinetics, electrokinetics, interfacial flows, wetting phenomena and biomolecular separation belong to our areas of work.

Picture: Sebastian Keuth


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

February 18, 2020

Tutorium „CFD-based simulation and optimization of microfluidic components“

This tutorium will equip students with the basic methodology needed to design and optimize microfluidic components on the computer. The course language will be English. Click here for details

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