Our research extends over a broad thematic spectrum and combines experimental, theoretical and numerical approaches. Nanoscale gas kinetics, electrokinetics, interfacial flows, wetting phenomena and biomolecular separation belong to our areas of work.
December 12, 2019
Nanoparticle-wall interactions in gases
A sphere moving in the vicinity of a wall experiences an increased fluid-dynamic drag force compared to motion far from solid boundaries, influencing the adsorption of particles at surfaces. For a small particle inside a gas, the fluid dynamics of such problems becomes very involved, since the Navier-Stokes equation is no longer applicable. We have computed the corresponding drag force on a particle and find that it is much lower than the one predicted by the Navier-Stokes equation.
Reference: P. Goswami, T. Baier, S. Tiwari, C. Lv, S. Hardt, and A. Klar, Drag force on spherical particle moving near a plane wall in highly rarefied gas. Journal of Fluid Mechanics 883, 47 (2020). DOI:10.1017/jfm.2019.921. Link: https://doi.org/10.1017/jfm.2019.921
June 7, 2019
Non-contact electrostatic manipulation of droplets on liquid-infused surfaces
Liquid-infused surfaces have unique liquid-repelling properties. In comparison with conventional superhydrophobic surfaces, liquid-infused surfaces provide stable wetting states and pronounced self-healing properties. We have established the manipulation of highly mobile droplets sitting on silicone-oil infused surfaces by exploiting a non-uniform electric field between a grounded substrate and a non-touching pin electrode placed above it. Droplets are attracted towards the pin electrode, and translational velocities can exceed 1 cm/s.
Reference: N. Sinn, M. T. Schür, and S. Hardt, No-contact electrostatic manipulation of droplets on liquid-infused surfaces: Experiments and numerical simulations, Applied Physics Letters 114 (2019), 213704. DOI: 10.1063/1.5091836
May 8, 2019
Evaporation of droplets on surfaces with wettability patterns
On a surface with hydrophilic and hydrophobic stripes, an evaporating droplet breaks up if the wettability contrast between the different regions is high enough. A liquid bridge forms on the hydrophobic stripe, and when the width of the bridge reaches a critical value, it becomes unstable and breaks up rapidly. We have studied this process based on experiments, numerical simulations, and a heuristic analytical model.
March 13, 2019
Review article on surface-tethered polymers in confinement
In a number of microfluidic applications, polymers are tethered to the microchannel walls. Even in situations where the channel diameter is significantly larger than the radius of gyration of the polymer, confinement effects can become important. Our recent review article gives an overview of the research results in this area.
March 6, 2019
On-demand production of femtoliter droplets in microchannels
We have developed a method to produce monodisperse droplets inside microchannels with volumes in the femtoliter range on demand. The method utilizes pulsed electric fields deforming the interface between an aqueous and an oil phase and pinching off droplets. It can be applied even to solutions with a zero-shear rate viscosity more than ten thousand-fold higher than that of water. The droplets may serve as biological reaction compartments.
Reference: M. Shojaeian, F. X. Lehr, H. U. Göringer, and S. Hardt, On-demand production of femtoliter drops in microchannels and their use as biological reaction compartments, Analytical Chemistry 91 (2019) 3484-3491. DOI: 10.1021/acs.analchem.8b05063
February 7, 2019
Tutorium “CFD-based simulation and optimization of microfluidic components”
At the beginning of the coming semester, we offer a Tutorium to all students interested in learning how to use CFD methods to design and optimize microfluidic components. Click here for details
Controlling the trajectories of nano/micro particles using light-actuated Marangoni flow
The ability to manipulate small objects and to produce patterns on the nano- and microscale is of great importance, both with respect to fundamentals and technological applications. The manipulation of particles with diameters of the order of 100 nm or below is a challenge because of their Brownian motion but also because of the scaling behavior of methods such as optical trapping. We have developed a method enabling the trapping and manipulation of nano- and microparticles based on interfacial flows controlled by visible light. The inherent advantages of this method are the linear scaling of the trapping force with the particle diameter and the fact that the force is less dependent on particle properties than in the case of conventional methods.
Reference: C. Lv, S. N. Varanakkottu, T. Baier, and S. Hardt, Controlling the trajectories of nano/micro particles using light-actuated Marangoni flow, Nano Letters 18 (2018), 6924−6930. DOI: 10.1021/acs.nanolett.8b02814
October 22, 2018
Electrophoresis of surface particles
The electrophoresis of particles immersed in a liquid is a well-studied phenomenon. However, what happens when a particle attached to a liquid surface translates along that surface driven by an electric field has been largely unknown. We have computed the electrophoretic mobility of a particle at the interface between two fluids with large viscosity contrast. For thin Debye layers, the Smoluchowki mobility is recovered. Generally, the mobility depends on the contact angle between the fluids and the particle. We have also calculated the interfacial deformation caused by Debye layer around the particle.
Reference: M. Eigenbrod, F. Bihler, and S. Hardt, Electrokinetics of a particle attached to a fluid interface: Electrophoretic mobility and interfacial deformation, Physical Review Fluids 3 (2018), 103701. DOI: 10.1103/PhysRevFluids.3.103701
October 8, 2018
Relaxation of surface-tethered polymers under moderate confinement
When long-chain polymers attached to a surface become stretched and the stretching force is released, they relax back to their coiled state within a characteristic time, denoted the relaxation time. We have shown that the polymers “feel” the presence of a second wall even if the distance between the two walls h is significantly larger than the radius of gyration of the polymers. As a result, the relaxation time increases. These results are relevant for microchannel flows where the channel walls are decorated with polymers.
Reference: J. Hartmann, T. Roy, K. Szuttor, J. Smiatek, C. Holm, and Steffen Hardt, Relaxation of surface-tethered polymers under moderate confinement, Soft Matter 14 (2018), 7926-7933. DOI: 10.1039/c8sm01246f
October 1, 2018
Stability of holes in liquid films
If you spill liquid over a surface, it forms a film with a thickness of the order of its capillary length. If the surface is bounded and you did not spill enough liquid, a hole will form. The stability of the hole depends on its size. There is a threshold size below which it collapses. For a water film, the collapse is dominated by inertia. We studied the stability and collapse of holes in liquid films in detail and compared the experimental results with mathematical models and simulations.
September 21, 2018
Thermophoresis of Janus particles at large Knudsen numbers
Thermophoresis, the motion of a particle along a thermal gradient, is exploited for deposition of aerosols on cooled surfaces. For non-symmetric particles it may be desirable to induce deposition with a preferred orientation of the particle. As a model system for this situation, we consider a spherical Janus particle having dissimilar reflective properties for gas molecules on its opposite hemispheres and investigate the interplay between rotational diffusion and thermophoretic motion on the orientation of the particle.
Reference: Tobias Baier, Sudarshan Tiwari, Samir Shrestha, Axel Klar, and Steffen Hardt, Thermophoresis of Janus particles at large Knudsen numbers, Phys. Rev. Fluids 3, 094202 (2018), DOI: 10.1103/PhysRevFluids.3.094202