Fachgebiet Nano- und Mikrofluidik

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

October 30, 2023

Oral presentation award for our PhD student Aaron Ratschow at ICOM2023

At the First Indian Conference on Micro Nano Fluidics, held 29th September – 1st October 2023 at IIT Madras, our PhD student Aaron Ratschow received a presentation award for his talk entitled “Gate electrodes can modulate nanofluidic potential energy landscapes”. The award was presented by Prof. Dimos Poulikakos, ETH Zurich.

Congratulations Aaron!

https://www.icom2023.in/

August 25, 2023

New group member: Alexander Wagner

A few days ago, Alexander Wagner joined our group. He will pursue his PhD and study wetting and optimization of grid structures.

Welcome Alexander!

August 21, 2023

Efficient pumping of liquids by temperature gradients

The standard method to pump a liquid through a channel or duct is to apply a difference in pressure between the inlet and outlet, i.e. a pressure gradient. It is also known that electrolyte solutions such as water can be pumped by applying a temperature gradient, an effect that is termed “thermoosmotic flow”. However, the flow velocities that can be reached using that principle are small, which has so far prevented widespread applications. On the nanoscale, boundary slip needs to be taken into account, which means that in some cases the liquid slips along the channel walls. We were able to derive analytical expressions for the thermoosmotic flow field in slit channels with boundary slip. For channels formed by polarized graphene surfaces, we predict a velocity enhancement factor of up to 250 compared to no-slip channel walls. This may open the door to applications of thermoosmotic flow in nanofluidics. The results were recently published in JFM Rapids.

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.

Welcome Pramodt!

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 (https://physics.aps.org/articles/v15/s174), where further information can be found.

 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.

Welcome Lisa!

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).

Welcome Doyel!

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.

Welcome Steffen!