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!


March 19, 2024

Towards controlled chemical reactions between minute samples

Performing chemical reactions between minute samples is the prerequisite for a number of industrial processes, for example high-throughput screening in pharmacological research. Ideally, one would be able to perform these reactions in a very controlled manner. Among others, it is desirable to bring two reagents in contact for a well-defined time, after which the reaction is stopped and its progress is analyzed. We have developed a method that could enable such a reaction control: Two samples, separated by a narrow spacer, are transported through a microchannel by electrophoresis, where an oscillatory electric field brings the samples in contact in a time-periodic manner. The results were published in the journal Analytical Chemistry.

March 01, 2024

Transformation of heat into electricity in nanochannels

About 70% of all the energy produced from sources such as power generators, factories, and homes is lost in the form of heat and dissipated to the environment. A technology that can convert only a small percentage of this energy into electricity could become a game changer. One promising option is to use nanopores or nanochannels filled with an electrolyte for energy conversion. We have identified a new energy-conversion mechanism that produces high thermovoltages and makes use of the fact that the concentration of charge carriers in nanochannels is temperature dependent. The results were published in the journal Physical Review Letters and highlighted on the TU Darmstadt homepage.

February 29, 2024

Disorder-to-order transition of long fibers in evaporating drops

Usually the world evolves towards increasing disorder, as expressed by the second law of thermodynamics. There are, however, some remarkable exceptions, i.e. systems that self-organize in such a way that their order increases. We have discovered one such system. When a liquid drop that contains a long fiber (much longer than the drop diameter) evaporates, the fiber evolves from an initially disordered configuration to an ordered one. For example, after the liquid has evaporated, the fiber is deposited on a surface with the shape of the number 8. These results were recently published in the journal Soft Matter.

February 1, 2024

New group member: Santanu Kumar Das

Today, Santanu Kumar Das joined our group as a postdoctoral researcher. He will focus on the modeling on simulation of thermoelectric energy conversion in nanochannels.

Welcome Santanu!

February 1, 2024

New group member: Oles Dubrovski

Today, Oles Dubrovski joined our group as a postdoctoral researcher. He will focus on the mathematical modeling of transport processes on small scales.

Welcome Oles!

December 6, 2023

High voltages in sliding drops

Water drops sliding on surfaces get electrically charged. How much? Up to several kilovolts! We demonstrate this surprising behavior in experiments. Further, we theoretically show that the reason lies in the surface potential, a fundamental property of solid-liquid-interfaces, which is electrostatically amplified in sliding drops. Our findings have strong implications for energy harvesting from sliding drops and enable a simple and inexpensive way of measuring surface potentials. These results were recently published in The Journal of Physical Chemistry Letters.

December 1, 2023

Electric charges influence wetting

Whether a liquid drop sticks to a surface or rolls off depends on contact angle hysteresis—the difference between the angles formed at the advancing and receding contact lines of a moving drop. While having been researched for a century, established theories have overlooked one essential contribution to contact angle hysteresis. We show that electric charges, spontaneously left on a surface by sliding water drops, can substantially influence contact angles through electrostatic interactions and thus hinder roll-off. The effect occurs for a wide range of surfaces and aqueous electrolytes. We explain the underlying mechanism with a quantitative theory. These results were recently published in  Physical Review Letters

November 28, 2023

Prof. Hardt’s talk at ICOM2023 now online at YouTube

The talk Prof. Hardt gave at the First Indian Conference on Micro Nano Fluidics is now available at YouTube, see:

October 30, 2023

Oral presentation award for our PhD student Aaron Ratschow at ICOM2023

At the First Indian Conference on Micro Nano Fluidics, held 29th September – 1st October 2023 at IIT Madras, our PhD student Aaron Ratschow received a presentation award for his talk entitled “Gate electrodes can modulate nanofluidic potential energy landscapes”. The award was presented by Prof. Dimos Poulikakos, ETH Zurich.

Congratulations Aaron!

August 25, 2023

New group member: Alexander Wagner

A few days ago, Alexander Wagner joined our group. He will pursue his PhD and study wetting and optimization of grid structures.

Welcome Alexander!

August 21, 2023

Efficient pumping of liquids by temperature gradients

The standard method to pump a liquid through a channel or duct is to apply a difference in pressure between the inlet and outlet, i.e. a pressure gradient. It is also known that electrolyte solutions such as water can be pumped by applying a temperature gradient, an effect that is termed “thermoosmotic flow”. However, the flow velocities that can be reached using that principle are small, which has so far prevented widespread applications. On the nanoscale, boundary slip needs to be taken into account, which means that in some cases the liquid slips along the channel walls. We were able to derive analytical expressions for the thermoosmotic flow field in slit channels with boundary slip. For channels formed by polarized graphene surfaces, we predict a velocity enhancement factor of up to 250 compared to no-slip channel walls. This may open the door to applications of thermoosmotic flow in nanofluidics. The results were recently published in JFM Rapids.

June 7, 2023

New group member: Pramodt Srinivasula

A few days ago, Pramodt Srinivasula joined our group as a postdoc. He will work on the design and simulation of microfluidic supply networks for organoids.

Welcome Pramodt!

May 22, 2023

Detection of minute molecular samples in microchannels

In the detection of minute (bio)chemical samples, electrophoretic schemes, i.e. sample transport due to electric fields, play a big role. Detecting even smallest sample amounts during electrophoretic transport is a big challenge. We have demonstrated that knowledge about the physics of electrophoretic transport can help reducing the detection limits by orders of magnitude. The data postprocessing scheme we have developed allows extracting signals from a noisy background even in cases where, based on conventional methods, no signal is discernable. These results were recently published in the journal Analytical Chemistry.

May 8, 2023

On superhydrophobic surfaces, surfactants prevent drag reduction

Theory predicts that, due to the large amount of air trapped in the grooves of a superhydrophobic surface, water can flow past it almost unimpeded. Surfactants, molecules that attach to the gas-liquid interface, can form a nearly incompressible layer at the interface, strongly affecting the flow in their vicinity. We study liquid flow over an array of narrow gas-filled grooves embedded in an otherwise planar surface, with the gas-liquid interface protruding above or below the plane. In the presence of surfactants, a recirculating flow develops at the gas-liquid interfaces, such that the drag on flow over such surfaces becomes much larger than at corresponding surfactant-free interfaces.

Baier, Influence of incompressible surfactant on drag in flow along an array of gas-filled grooves, Phys. Rev. Fluids, 8, 4, 044002, (2023).

January 9, 2023

Individually controllable nanopumps

We have explored a novel nanopumping concept that is based on electrokinetic flow through a conical nanopore equipped with a gate electrode. In contrast to other electrokinetic nanopumps, these pumps are individually controllable. Such enhanced control of the nanoworld may prove beneficial for a number of applications, among others DNA sequencing. Our work was highlighted on the web pages of the American Physical Society (, where further information can be found.

A. D. Ratschow, D. Pandey, B. Liebchen, S. Bhattacharyya, and S. Hardt, Resonant nanopumps: ac gate voltages in conical nanopores induce directed electrolyte flow, Physical Review Letters 129 (2022), 264501