Institute for Nano- and Microfluidics

October 21, 2024

Electrophoresis of uncharged particles

Electrophoresis is one of the most important separation techniques in (bio)chemical analytics. It relies on the motion of charged molecules or particles that are suspended in a liquid under application of an electric field. We have shown that if the material of the suspended particle has a very high dielectric permittivity and if a charge-asymmetric electrolyte is considered (for example, a monovalent cation in combination with a multivalent anion), it will move in an applied electric field even if it bears no charge. Using the same ideas, we have shown that an electroosmotic flow through a channel with highly polarizable walls is generated even if there is no net charge at the channel walls. The corresponding flow velocities are high, which is why the effect could be utilized for pumping through nanochannels. The results were published in the Journal of Fluid Mechanics.

September 9, 2024

Lisa Bauer wins best paper award at ICTAM 2024

Recently, Lisa won the best paper award at the 26th International Congress of Theoretical and Applied Mechanics in the category fluid mechanics. The ICTAM conference series is probably the most important conference series in the field of classical mechanics worldwide, being held every four years. Without any doubt, winning the best paper award at such a conference is a great achievement.

Congratulations Lisa!

September 6, 2024

New lecture: Electrokinetics and Electrohydrodynamics

Beginning with the winter term 2024/2024, we offer a new M.Sc. course entitled “Electrokinetics and Electrohydrodynamics”. The aim of the lecture is to convey important basic knowledge in the context of the growing importance of electrical energy conversion systems in Mechanical Engineering. In many of these systems, liquids (especially electrolytes) are used which are affected by electric fields. The transport phenomena in such systems significantly determine their performance characteristics. In this new lecture, corresponding theoretical concepts based on the physics of interactions between fluid mechanics and electrostatics are covered.

July 17, 2024

Vibrations reduce the surface tension of liquids

If a liquid volume is vibrated with sufficient amplitude, capillary waves emerge on its surface, termed Faraday waves. With increasing excitation amplitude, these waves become more and more chaotic. Interestingly, a liquid volume with chaotic Faraday waves behaves as if it has a reduced surface tension, which we have shown using a combination of experiments and theoretical models. In this way, we have been able to map a complex time-dependent system to a much simpler system, i.e., a steady-state capillary surface. This work was recently published in Physical Review Letters and has been highlighted on the homepage of TU Darmstadt.

July 7, 2024

Our work on the cover of Soft Matter

In a team with colleagues from the University of Stuttgart and the Max Planck Institute for Polymer Research we studied the surface charges left behind by sliding water drops. We developed a measuring method based on image charge detection which helped us corroborate our theoretical understanding of the charge separation mechanism. The work is featured on the cover of the journal Soft Matter.

July 1, 2024

New group member: Zhichao Deng

Recently Zhichao Deng joined our group as a postdoctoral researcher. He will experimentally investigate light-induced flow patterns.

Welcome Zhichao!

June 3, 2024

How nature charges water drops

Water drops sliding on a surface get electrically charged. Known for 30 years, this effect holds potential for energy harvesting but also damages semiconductors that are repeatedly washed throughout their production. We now provide a long-missing theoretical explanation for this charge separation. Our theory uncovers why the effect is predominantly observed on water-repelling surfaces and why it surprisingly decreases at higher velocities. Our work has been highlighted in the American Physical Society’s Physics Magazine and published in Physical Review Letters.

May 22, 2024

New group member: Shuyan Deng

Yesterday Shuyan Deng joined our group as a postdoctoral researcher. She will work on the modeling and simulation of microscale flow phenomena.

Welcome Shuyan!

May 7, 2024

Trapping individual nanoobjects with gate electrodes

Life is built from nanoscale biological objects. Thus, controlling their location is imperative for fundamental research and applications in the life sciences. However, Brownian motion, random spatial fluctuations of small particles due to their thermal energy, makes their precise localization difficult. One strategy to overcome this obstacle uses electrostatic interactions. Nanoparticles in aqueous solutions, as well as the walls of nanofluidic devices, are usually negatively charged and repel each other. By creating indents in the walls of nanofluidic devices, repulsion from the flat walls, as well as the indents can be used to restrict the location of nanoparticles. Together with a team at IBM Research Europe and ETH Zurich), we have shown that gate electrodes can be used to control these electrostatic interactions. The results, recently published in The Journal of Physical Chemistry Letters open routes towards real-time controllable nanoparticle traps.

May 3, 2024

New group member: Florian Stoll

A few days ago, Florian Stoll joined our group. He will pursue his PhD and experimentally study the transport processes through liquid pores.

Welcome Florian!

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: www.youtube.com/watch?v=Eig9nNTVLUE