Institute for Nano- and Microfluidics

Welcome at the Institute for Nano- and Microfluidics!

We deal with transport phenomena in fluids on the nano- and micrometer scale. We are particularly interested in basic research with the intention of paving the way for novel applications. Our approach is based on a “bottom-up” strategy, i.e. the research is knowledge-driven. Breaking new scientific ground fascinates us, but we keep applications in mind. These can be in different fields such as sustainability, energy conversion, process engineering or (bio)chemical analytics.

Our research covers a broad range of topics and combines experimental, theoretical and numerical approaches. Our fields of work include gas kinetics on the nanoscale, transport processes in electrolyte solutions and at liquid interfaces, wetting phenomena, and separation processes for biomolecules.

In teaching, we offer master's level courses that reflect our research approach. This means that great importance is attached to the understanding of phenomena and processes, but these are also considered in an application context.

If we have aroused your interest, we would be pleased to hear from you!

Picture: Sebastian Keuth

News

September 21, 2021

New group member: Satarupa Dutta

A few days ago, 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, 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 (https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.225901), 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: https://www.tu-darmstadt.de/universitaet/aktuelles_meldungen/einzelansicht_320064.de.jsp

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 https://physics.aps.org/articles/v14/s57.

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.