Nano- and Microfluidics
Ejection of droplets from a liquid jet
Formation of small droplets inside larger droplets
Microfluidic chip with two-phase flow for DNA or protein separation in an electric field
Wetting transition on a superhydrophobic surface
Electroosmotic flow along a superhydrophobic surface induced by a gate electrode
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!
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.
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). https://doi.org/10.1103/PhysRevFluids.8.044002
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. https://physics.aps.org/articles/v15/s174
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
December 6, 2022
When a drop transforms into a bubble
Drops impacting on surfaces have been studied intensely during the past decades. It thus came as a surprise to us when we discovered a mode of drop impact that has remained undiscovered up to now. We let water drops impact on microporous membranes through which a gas discharges, for which we identified four distinct impact modes. In the most spectacular impact mode, a drop gets in contact with the membrane surface and forms a three-phase contact line away from the center of impact. The contact line remains pinned, while the gas flow through the membrane pushes the liquid surface away from it. As a result, the drop transforms into a bubble that remains attached to the membrane. When we work with a surfactant solution instead of water, the drops transform to large long-lived bubbles.
L. Weimar, L. Hu, T. Baier, and S. Hardt, Drop impact on a sticky porous surface with gas discharge: transformation of drops into bubbles, Journal of Fluid Mechanics 953 (2022), A6.
November 7, 2022
New group member: Lisa Bauer
A few days ago, Lisa Bauer joined our group. She will pursue her PhD and study dynamic behavior of fiber-laden drops.
October 31, 2022
Incompressible surfactants on the move
Even relatively small amounts of surfactants, molecules that attach to the interface between two fluids, can behave nearly incompressible at the interface. For liquid flowing over an elongated rectangular gas-filled cavity embedded in a planar wall, this incompressibility may render the interface immobile as long as the interface remains planar. By adjusting the gas-pressure in the cavity, the gas-liquid interface can be deflected above or below the planar wall. We find that in this case liquid flowing over the cavity sets the interface in motion, inducing a recirculating flow pattern at the interface.
Baier, Hardt, Shear flow over a surface containing a groove covered by an incompressible surfactant phase, J. Fluid Mech., 949, A34 (2022), https://doi.org/10.1017/jfm.2022.775
September 8, 2022
Tuning the wavelength of electrically induced waves
When a time-varying electric field acts on the interface between two immiscible liquids, characteristic waves form at the interface. A similar thing happens when two superposed liquids are vibrated using a shaker. These interface waves form in a self-organization process, and up to now, it was unknown how to tune their wavelength. We have found a way to exactly do that, as we show in a recently published paper: We superpose the time-varying field that drives the waves with a constant field. The results nicely agree with the predictions of a theoretical model we have developed.
S. Dehe, M. Hartmann, A. Bandopadhyay, and S. Hardt, Controlling the electrostatic Faraday instability using superposed electric fields, Phys. Rev. Fluids 7 (2022), L082002
September 7, 2022
New group member: Doyel Pandey
A few days ago, Dr. Doyel Pandey joined our group. She holds a PhD from the Indian Institute of Technology Kharagpur and will study thermally-induced transport processes in nanochannels, funded by a French-German co-operation project (ANR-DFG).
August 31, 2022
Phase transitions in evaporating drops
When a sessile aqueous drop containing different types of polymers evaporates, a demixing phase transition can occur. This means that small droplets nucleate inside the initially homogeneous polymer solution, which happens when the drop contains two polymer species that “like to stay away from each other”. During this complex phase separation process, a number of intriguing physical processes are observed. We describe and explain some of these phenomena in a recently published paper.
A. May, J. Hartmann, and S. Hardt, Phase separation in evaporating all-aqueous sessile drops, Soft Matter 18 (2022) 6313-6317.
August 11, 2022
New group member: Steffen Bißwanger
A few days ago, Steffen Bißwanger joined our group. He will pursue his PhD and study channel flows of ternary liquid mixtures in which phase change occurs.