Nano- und Mikrofluidik
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
Deformation of a fluid interface under an electric field
Herzlich willkommen am Fachgebiet Nano- und Mikrofluidik!
Wir beschäftigen uns mit Transportphänomenen in Fluiden auf der Nano- und Mikrometerskala. Dabei interessiert uns besonders die Erforschung von Grundlagen mit der Intention, den Weg für neuartige Anwendungen zu bereiten. Unser Ansatz beruht auf einer „Bottom-up“-Strategie, d. h. die Forschung ist erkenntnisgetrieben. Wissenschaftliches Neuland zu betreten fasziniert uns, aber dabei behalten wir Anwendungen im Blick. Diese können auf unterschiedlichen Gebieten wie beispielsweise Nachhaltigkeit, Energiewandlung, Verfahrenstechnik oder der (bio)chemischen Analytik liegen.
Unsere Forschung erstreckt sich über ein breites Themenspektrum und kombiniert experimentelle, theoretische und numerische Ansätze. Zu unseren Arbeitsgebieten gehören Gaskinetik auf der Nanoskala, Transportprozesse in Elektrolytlösungen und an Flüssigkeitsgrenzflächen, Benetzungsphänomene und Trennprozesse für Biomoleküle.
In der Lehre bieten wir Veranstaltungen auf Masterniveau an, in denen sich unser Forschungsansatz wiederspiegelt. Dies bedeutet, dass dem Verständnis von Phänomenen und Prozessen eine große Bedeutung zukommt, diese aber auch im Anwendungskontext betrachtet werden.
Falls wir Ihr Interesse geweckt haben, würden wir uns über eine Kontaktaufnahme freuen!
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),
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.
March 24, 2022
Electrically-induced Faraday waves
It is well-known that wave patterns (so-called Faraday waves) emerge on a vibrated liquid film. Not only mechanical forces can induce such wave patterns, but also electric forces, for example when keeping the liquid in a parallel-plate capacitor to which an AC voltage is applied. Remarkably, electrically induced Faraday waves have hardly been studied experimentally, although electric forcing offers options to control the wave patterns not available with mechanical forcing. We have studied the wave patterns of the electrically induced Faraday instability for the first time and compared them to theoretical predictions accounting for viscosity effects, with the result that there is a good agreement between experiments and theory.
S. Dehe, M. Hartmann, A. Bandopadhyay, and S. Hardt, The spatial structure of electrostatically forced Faraday waves, Journal of Fluid Mechanics 939 (2022), A6.
March 10, 2022
New group member: Salar Farrokhi
A few days ago, Salar Farrokhi joined our group. He will pursue his PhD and study dynamic wetting on microstructured surfaces.
January 12, 2022
Coalescence control using electric fields
Controlling the coalescence of droplets is a key requirement in microfluidics. In that context, “controlling” often means avoiding the undesired coalescence of droplets. For this purpose, usually surfactants are employed, which, however, are detrimental in a number of applications. We have shown how droplet coalescence can be suppressed without surfactants simply by placing the droplets in a homogeneous electric field. The electric field redistributes the charges in such a way that a repulsive force between two nearby droplets is created. We believe that this principle may find widespread applications in droplet microfluidics wherever surfactants need to be avoided.
J. Hartmann, M. T. Schür, and S. Hardt, Manipulation and control of droplets on surfaces in a homogeneous electric field, Nature Communications 13 (2022), 289
January 5, 2022
From Taylor cones to surface dimples
It is well known that fluid interfaces disintegrate under sufficiently strong electric fields, leading to electrohydrodynamic tip streaming, often in form of a Taylor cone. However, when we studied the influence of a local electric field on an oil-water interface, an alternative deformation mode emerged: Here, the interface is pushed away from the electrode, and additional cone structures form at the rim of the dimple. Using experimental and numerical methods, we show that small droplets inside the oil phase play a crucial role in the dimple formation. They induce a background flow, which in turn perturbs the interface.
Sebastian Dehe and Steffen Hardt, Deformation modes of an oil-water interface under a local electric field: From Taylor cones to surface dimples, Phys. Rev. Fluids 6 (2021), 123702.
October 19, 2021
Partial droplet coalescence – where does it end?
When an electric field derives a water droplet to an interface between oil and water, under certain circumstances partial coalescence is observed. This means that instead of merging with the water volume, the droplet bounces back, whereupon it looses a part of its volume. The product droplet can again be driven to the oil-water interface, and the whole process can be repeated, resulting in sucessively smaller droplets. We have performed corresponding experiments with a large number of successive partial-coalescence events and could produce droplets with diameters of about 400 nm. The fact that we could not observe any smaller droplets is probably due to the optical resolution limits of our microscope. Would this process still work for droplet diameters of a few nanometers?
M. Shojaeian and S. Hardt, Manipulation of single sub-femtolitre droplets via partial coalescence in a direct-current electric field, Flow 1 (2021), E12.
New group member: Arman Sadeghi
A few days ago, Professor Arman Sadeghi joined our group. Before that, he worked as Associate Professor at the University of Kurdistan, Iran. He is funded by the Alexander von Humboldt Foundation in terms of a Humboldt Research Fellowship.
October 10, 2021
New group member: Qingwen Dai
A few days ago, Dr. Qingwen Dai joined our group. Before that, he worked as a lecturer at Nanjing University of Aeronautics & Astronautics, China. He is funded by the Alexander von Humboldt Foundation in terms of a Humboldt Research Fellowship.
Article published in Annual Review of Fluid Mechanics
Recently, an article entitled “Flow and Drop Transport Along Liquid-Infused Surfaces”, authored by Steffen Hardt and Glen McHale, was published in the Annual Review of Fluid Mechanics. In this article, recent developments related to liquid-infused surfaces are reviewed from a fluid mechanics perspective. It covers a spectrum ranging from single-phase flow along liquid-infused surfaces to dynamic wetting processes. A preprint of the article can be found here:
September 21, 2021
New group member: Satarupa Dutta
A few days ago, Dr. 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.