Fachgebiet Nano- und Mikrofluidik

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!

News

May 3, 2022

New group member: Behnaz Shamsizadeh

A few days ago, Behnaz Shamsizadeh joined our group. She will pursue her PhD and study microfluidic network structures with the LOEWE project „Flow for Life“.

https://www.tu-darmstadt.de/flowforlife/about_ffl/index.en.jsp

Welcome Behnaz!

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.

Welcome Salar!

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.

October 18,2021

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.

Welcome Arman!

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.

Welcome Qingwen!

September 30,2021

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:

https://www.annualreviews.org/doi/abs/10.1146/annurev-fluid-030121-113156

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.

Welcome Satarupa!

August 24, 2021

Hydrodynamic dispersion in Hele-Shaw flows

In the 1950s, Taylor and Aris published their landmark results on the spreading of a dissolved species in unidirectional flow, i.e., in a tube or a channel, referred to as Taylor-Aris dispersion. We have considered flows between parallel-plates (so-called Hele-Shaw flows), which are no longer unidirectional, but may exhibit complex patterns owing to inhomogeneous wall boundary conditions. For such scenarios, we have asked how a dissolved species may spread. We have been able to reduce the 3D advection-diffusion equation for the concentration field to a 2D dispersion equation that may be viewed as a generalization of Taylor-Aris dispersion. We believe that in the future this equation could find widespread use in simplifying simulations of hydrodynamic dispersion in microfluidics.

S. Dehe, I. S. Rehm, and S. Hardt, Hydrodynamic dispersion in Hele-Shaw flows with inhomogeneous wall boundary conditions, Journal of Fluid Mechanics 925 (2021), A11.

August 6, 2021

New group member: Rajkumar Sarma

A few days ago, Dr. Rajkumar Sarma joined our group. He recently obtained his PhD at the Indian Institute of Technology Guwahati and will study thermoelectric energy conversion in nanochannels in the framework of the EU project TRANSLATE.

Welcome Rajkumar!

June 22, 2021

Start of EU project TRANSLATE

Recently, the EU project TRANSLATE, funded under the FET-Open scheme of the EU, was launched. Heavily relying on results we published a few years ago (https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.225901), this project aims at establishing thermoelectric energy converters based on electrolyte-filled nanochannels. This could pave the way to the widespread use of inexpensive energy conversion devices made from highly abundant materials. Apart from our group, partners from the University College Cork (Ireland), the University of Latvia and the Spanish company Cidete form the TRANSLATE team. Further information can be found here: https://www.tu-darmstadt.de/universitaet/aktuelles_meldungen/einzelansicht_320064.de.jsp

May 7, 2021

Cloaking and shielding objects in a fluid flow

Among the best-known technologies deployed by Star Trek’s starships are their invisibility-inducing cloaking devices and their shields. Together with co-operation partners from the Technion, Israel, and IBM Research Europe we have developed a cloaking/shielding device for objects in microfluidic channels/chambers instead of starships. In cloaking mode, an object leaves the fluid flow around it undisturbed, and in shielding mode, the hydrodynamic forces on an object are eliminated. This principle may find a number of applications, for example in the manipulation of soft objects such as cells. See also https://physics.aps.org/articles/v14/s57.

E. Boyko, V. Bacheva, M. Eigenbrod, F. Paratore, A. D. Gat, S. Hardt, and M. Bercovici, Microscale hydrodynamic cloaking and shielding via electro-osmosis, Physical Review Letters 126 (2021), 184502.

April 22, 2021

Breakup of a liquid bridge on a solid surface

Liquid bridges that become unstable and break up are found in numerous situations, for example when a droplet pinches off from a liquid reservoir. We have studied the breakup dynamics of water bridges wetting a hydrophobic surface using experiments and simulations. The dynamics is governed by a balance of inertial and capillary forces. We find that the liquid bridge decays into droplets in very much the same way as a free liquid bridge.

M. Hartmann, M. Fricke, L. Weimar, D. Gründing, T. Marić, D. Bothe, and S. Hardt, Breakup dynamics of capillary bridges on hydrophobic stripes, International Journal of Multiphase Flow 140 (2021),103582.

April 16, 2021

Protein separation at liquid-liquid interfaces

The separation of proteins according to specific properties such as size plays an eminent role in many processes of the biotech industry. We have recently demonstrated a new separation process for proteins in a microfluidic device. First, a protein mixture is electrophoretically transported towards a liquid-liquid interface. The interface represents a transport resistance to the proteins, such that some species adsorb at the interface more easily than others. Protein separation can be accomplished if one species crosses the interface, while the other gets adsorbed, which was demonstrated in the paper referred to below. We believe that in the future this new method of protein separation could extend the spectrum of industrial separation processes.

F. Gebhard, J. Hartmann and S. Hardt, Interaction of proteins with phase boundaries in aqueous two-phase systems under electric fields, Soft Matter 17 (2021), 3929-3936.