Institute for Nano- and Microfluidics

April 28, 2025

Spontaneous symmetry breaking as a motor for Leidenfrost drops

Almost everyone is familiar with the Leidenfrost effect: a drop of water on a hotplate does not touch the surface, but is held at a distance by a thin film of steam, which gives the drop a high degree of mobility. However, the movement of the drop on the hotplate is undirected and erratic. About two decades ago, it was discovered that a Leidenfrost drop moves in a directional and controlled manner on a surface with the structure of a ratchet. The reason is that the asymmetry of the ratchet ensures that the vapor below the drop escapes in a preferred direction, which propels the drop. In a co-operation with Cunjing Lv and his group, a professor at Tsinghua University in China and a former postdoc of our group, we found a similar drop motion on completely symmetric surface structures, e.g., on metal surfaces with parallel grooves. Our theoretical model explains this effect: Once set in motion, the liquid surface near the hot metal surface is asymmetrically deformed and thus provides the permanent drive for the drop. In this case, it is not the solid surface but the liquid surface that forms the ratchet. The results were published in the journal Science Advances. In a more general context, this phenomenon is known as spontaneous symmetry breaking: An initially symmetric system dynamically evolves into a state in which the symmetry is broken. The principle is so general that the findings from the experiments with Leidenfrost drop could be used to develop new concepts for transporting liquids on the millimeter or micrometer scale.

April 4, 2025

Impacting drops do not splash when they are charged

Drops hitting surfaces at high velocity often splash and eject smaller droplets. This everyday phenomenon can cause challenges in technical applications like surface coating or printing. In a team with colleagues from China and Switzerland, we have shown that a small electric drop charge can keep them from splashing. Charges in the drop are electrostatically attracted to the surface and stabilize the drop. Our theoretical model accurately predicts this behavior. The work has been highlighted in the American Physical Society’s Physics Magazine and published in Physical Review Letters, marked as Editors’ Suggestion.

March 7, 2025

New group member: Merete Seyfried

Recently, Merete Seyfried joined our group as a PhD student. She will study thermoelectric energy conversion in electrolyte-filled nanochannels.

Welcome Merete!

February 3, 2025

Review article on liquid slide electrification

When a drop slides along a solid surface, it often leaves behind charge and accumulates the countercharge. This ubiquitous phenomenon challenges accepted theories of dynamic wetting. In cooperation with partners from the Max Planck Institute for Polymer Research and the University of Stuttgart, we have recently published a review article on liquid slide electrification in the journal Soft Matter. The article not only reviews the physical mechanisms underlying slide electrification and the consequences of this phenomenon, it also highlights open questions and suggests directions for future research in this field.

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.