Gravity-driven films over topography

We study the impact of bottom undulations on the film flow to identify dynamic flow regimes that can be used for process optimization. The flow of a viscous film along a corrugated substrate is encountered in many industrial and environmental systems such as debris and lava flows and avalanches or in coating applications, heat exchangers, distillation columns, tube bundle condensers, evaporators and protective coatings and many others.In the case of the flow over a flat incline, the film is steady with a parabolic velocity profile below a critical Reynolds number. Beyond this critical Reynolds number, it becomes unstable with respect to kinematic surface waves exhibiting complex dynamics. Substrate undulations do not only alter the film flow on a quantitative level like retardation of instabilities, but also may provoke flow regimes not encountered on even surfaces. These include: resonance with bottom undulations, three-dimensional surface instabilities and different types of hydraulic jumps. The pictures show an example for unsteady flow in direct comparison. At sufficiently high steepness of the bottom contour or high inertia, eddies may form in the troughs. They can be manipulated to control material exchange and, choosing the right parameters, they can even be suppressed.

Impact of undulated bottom contours on film flows. Left: flat bottom, right: undulated bottom. Apart from the bottom, all other parameters are identical. On the flat incline, surface waves are generated. The undulated substrate yields a steady three-dimensional pattern close to the inflow and highly disordered unsteady flow below.

Contact: Andreas Wierschem
Telefon: +49 (0) 9131/85-29566
Fax: +49 (0) 9131/85-29503

Selected publications:

A. Wierschem,T. Pollak, C. Heining, N. Aksel, Suppression of eddies in films over topography,Physics of Fluids 22, 113603 (2010) (DOI: 10.1063/1.3504374)
C. Heining, V. Bontozoglou, N. Aksel, A. Wierschem, Nonlinear resonance in viscous films on inclined wavy planes, International Journal of Multiphase Flow 35, 78–90 (2009) (DOI: 10.1016/j.ijmultiphaseflow.2008.07.005)
A. Wierschem, V. Bontozoglou, C. Heining, H. Uecker, N. Aksel, Linear resonance in viscous films on inclined wavy planes, International Journal of Multiphase Flow 34, 580-589 (2008) (DOI: 10.1016/j.ijmultiphaseflow.2007.12.001)
A. Wierschem, C. Lepski, N. Aksel, Effect of long undulated bottoms on thin gravity-driven films, Acta Mechanica 179, 41-66 (2005) (DOI: 10.1007/s00707-005-0242-2)
A. Wierschem, N. Aksel, Influence of inertia on vortices created in films creeping over strongly undulated substrates, Physics of Fluids 16, 4566-4574 (2004) (DOI: 10.1063/1.1811673)
A. Wierschem, N. Aksel, Hydraulic jumps and standing waves in gravity-driven flows of viscous liquids in wavy open channels, Physics of Fluids 16, 3868-3877 (2004) (DOI: 10.1063/1.1789431)
M. Scholle, A. Wierschem, N. Aksel, Creeping films with vortices over strongly undulated bottoms, Acta Mechanica 168, 167-193 (2004) (DOI: 10.1007/s00707-004-0083-4)
A. Wierschem, N. Aksel, Instability of a liquid film flowing down an inclined wavy plane, Physica D 186, 221-237 (2003) (DOI: 10.1016/S0167-2789(03)00242-2)
A. Wierschem, M. Scholle, N. Aksel, Vortices in film flow over strongly undulated bottom profiles at low Reynolds numbers, Physics of Fluids 15, 426-435 (2003) (DOI: 10.1063/1.1533075)

Phase transitions under high pressure

We study liquid-solid phase transitions of food and pharmaceutical constituents at high pressures. Many of these materials are polymorphic, i.e. they can form different crystal arrangements. Their phase transitions and their crystal structures are of great importance for their handling, different production steps and for the final product. Usually, they are studied under atmospheric pressure conditions. However, in recent years, pressure treatment for instance of food has become increasingly popular. Thus, melting curves, crystallization and its dynamics have to be known to efficiently run these processes. The crystallization kinetics can be subdivided into different stages. After crossing the melting curve, it takes a certain period of time, the induction time, until the first nuclei form. In a subsequent step, these nuclei grow at a definite rate. We detect melting, induction times, and crystal nucleation and growth rates at pressures up to about 500 MPa in situ with optical techniques.

Single pressure-induced triolein crystal seen with a polarization microscope at a pressure of 300 MPa. Diameter: 700 µm.

Contact: Andreas Wierschem
Telefon: +49 (0) 9131/85-29566
Fax: +49 (0) 9131/85-29503


Selected publications:

M. Nagel, A. Wierschem, C. Rauh, A. Delgado, Optical measurement of the pH value of intransparent samples under high pressure, High Pressure Research 35, 162–169 (2015) (DOI: 10.1080/08957959.2015.1027202)
P. Ferstl, C. Eder, W. Ruß, A. Wierschem, Pressure induced crystallization of triacylglcerides, High Pressure Research 31, 339–349 (2011) (DOI: 10.1080/08957959.2011.582870)
D. B. Tefelski, L. Kulisiewicz, A. Wierschem, A. Delgado, A. J. Rostocki, R. M. Siegozyński, The Particle Image Velocimetry method in the study of the dynamics of phase transitions induced by high pressure in triolein and oleic acid,High Pressure Research 31, 178–185 (2011) (DOI: 10.1080/08957959.2010.551121)
P. Ferstl, S. Gillig, C. Kaufmann, C. Dürr, C. Eder, A. Wierschem, W. Ruß, Pressure induced phase transitions in triacylglycerides, High-Pressure Bioscience and Biotechnology 1189, 62-67 (2010) (DOI: 10.1111/j.1749-6632.2009.05179.x)


Narrow-gap rheometry and biorheology

Based on commercial rotational rheometers, we set up narrow-gap rheometers with a precision in gap width of up to ±0.7 µm for unidirectional shear and even higher for oscillatory studies, which is an improvement by more than an order of magnitude compared to standard rheometers. This enables studying samples at gap widths of a few µm, which is advantegeous of many kinds:

  • Small amount of sample (some 10 µl)
  • Viscosity and normal-stress differences at high shear rates (up to 105 s-1)
  • Measurementing low-viscous samples
  • Studying the impact of geometrical constraints
  • Studying average properties of cells in monolayers
  • Studying load and adhesion limits of cells in shear flows in their natural, low-viscous environment
  • ...

Rheometer new setup with sensor inside

Current projects:

  • Continuous improvement of narrow-gap rheometers
  • The second Newtonian regime of polymer solutions
  • Average viscoelastic properties of cells

Contact: Andreas Wierschem
Telefon: +49 (0) 9131/85-29566
Fax: +49 (0) 9131/85-29503


Selected publications:

H. Dakhil, H. Do, H. Hübner, A. Wierschem, Measuring the adhesion limit of fibronectin for fibroblasts with a narrow-gap rotational rheometer, Bioprocess and Biosystems Engineering (accepted) (DOI: 10.1007/s00449-017-1868-x)
H. Dakhil, D. F. Gilbert, D. Malhotra, A. Limmer, H. Engelhardt, A. Amtmann, J. Hansmann, H. Hübner, R. Buchholz, O. Friedrich, A. Wierschem, Measuring average rheological quantities of cell monolayers in the linear viscoelastic regime, Rheologica Acta 55, 527-536 (2016) (DOI: 10.1007/s00397-016-0936-5)
D. Kokkinos, H. Dakhil, A. Wierschem, H. Briesen, A. Braun, Deformation and rupture of Dunaliella salina at high shear rates without the use of thickeners, Biorheology 53, 1-11 (2016) (DOI: 10.3233/BIR-15057)
H. Dakhil, A. Wierschem, Measuring low viscosities with a rotational rheometer in a thin-gap parallel-disk configuration, Applied Rheology 24, 63795 (2014) (DOI: 10.3933/APPLRHEOL-24-63795)

Rheology of new materials

In cooperation with material scientists, we characterize their new materials.

Typical butterfly pattern obtained in viscoelastic wormlike micellar solution under shear

Contact: Andreas Wierschem
Telefon: +49 (0) 9131/85-29566
Fax: +49 (0) 9131/85-29503


Selected publication:

L. Niklaus, S. Tansaz, H. Dakhil, M. Pröschel, M. Lang, M. Kostrzewa, P. B. Coto, R. Detsch, U. Sonnewald, A. Wierschem, A. R. Boccaccini, R. D. Costa, Micropatterned down-converting coating for white bio-hybrid light-emitting diodes, Advanced Functional Materials 27, 1601792 (2017) (DOI: 10.1002/adfm.201601792)
L. Niklaus, H. Dakhil, M. Kostrzewa, P. Branda-Coto, U. Sonnewald, A. Wierschem, R. D. Costa, Easy and versatile coating approach for long-living white hybrid light-emitting diodes, Materials Horizons 3, 340 - 347 (2016) (DOI: 10.1039/c6mh00038j)
M. Kostrzewa, A. Delgado, A. Wierschem, Particle settling in micellar solutions of varying concentration and salt content, Acta Mechanica 227, 677-692 (2016) (DOI: 10.1007/s00707-015-1472-6)
S. Herrmann, M. Kostrzewa, A. Wierschem, C. Streb, Polyoxometalate-based ionic liquids (POM-ILs) as self-repairing acid-resistant corrosion protection, Angewandte Chemie International Edition 53, 13596–13599 (2014) (DOI: 10.1002/anie.201408171)

Onset of particle motion

Predicting the onset of granular motion by a shear flow has been the subject of numerous studies over the last century. It is very important on the initial process of several natural situations like sediment transport in rivers and coastal flows, granular beds erosion or dune formation. Moreover, it is also of vital importance in a wide variety of industrial operations including cleaning of surfaces, e.g. filtration and production facilities in food and pharmaceutical industries, and oil extraction. Further applications for laminar flow induced removal of solid particles from an arranged substrate are considered in areas such as microfluidics. The template-assembly of microparticles using microfluidics or the assembly of coupled microdevices in confined structured geometries are examples of further applications.

Microscopic top view of regular substrates made from identical spherical glass beads of 400 μm diameter. (a) Hexagonal configuration; (b) quadratic configuration on a mesh, gap between particles: 14 μm; (c) quadratic  configuration on a mesh, gap between particles: 109 μm.

We study incipient particle motion experimentally, numerically and analytically. At small particle Reynolds numbers, the critical Shields parameter, which compares the characteristic shear stress acting on the particle to the resistant specific particle weight that retains it in place, is independent from the particle density and from inertia. However, it depends significantly on the geometry of the substrate and on the size ratio of the particles. Our model describes properly the geometrical impact on the critical Shields number observed experimentally and numerically. Besides the substrate itself, the incipient motion is also strongly affected by neighboring particles in a non-monotonous way.

Contact: Andreas Wierschem
Telefon: +49 (0) 9131/85-29566
Fax: +49 (0) 9131/85-29503


Selected publications:

J. R. Agudo, J. Han, J. Park, S. Kwon, S. Loekman, G. Luzi, A. Delgado, A. Wierschem, Characterizing the incipient particle motion as a function of the bed geometry; from the laminar to the fully turbulent flow regime, Journal of Visualized Experiments (accepted)
J. R. Agudo, C. Illigmann, G. Luzi, A. Laukart, A. Delgado, A. Wierschem, Shear-induced incipient motion of a single sphere on uniform substrates at low particle Reynolds numbers, Journal of Fluid Mechanics 825, 284-314 (2017) (DOI: 10.1017/jfm.2017.370)
J. R. Agudo, G. Luzi, J. Han, M. Hwang, J. Lee, A. Wierschem, Detection of granular motion using image processing with particular emphasis on rolling motion, Review of Scientific Instruments 88, 051805 (2017) (DOI: 10.1063/1.4983054)
A. Amon, P. Born, K. Daniels, J. Dijksman, K. Huang, D. Parker, M. Schröter, R. Stannarius, A. Wierschem, Focus on imaging methods in granular physics, Review of Scientific Instruments 88, 051701 (2017) (DOI: 10.1063/1.4983052)
J. R. Agudo, S. Dasilva, A. Wierschem, How do neighbors affect incipient particle motion in laminar shear flow? Physics of Fluids 26, 053303 (2014) (DOI: 10.1063/1.4874604)
J. R. Agudo, A. Wierschem, Incipient motion of a single particle on regular substrates in laminar shear flow, Physics of Fluids E 24, 093302 (2012) (DOI: 10.1063/1.4753941)
C. Groh, A. Wierschem, N. Aksel, I. Rehberg, C. A. Kruelle, Barchan dunes in two dimensions: experimental tests of minimal models, Physical Review E 78, 021304 (2008) (DOI: 10.1103/PhysRevE.78.021304)
A. Wierschem, C. Groh, I. Rehberg, N. Aksel, C. A. Kruelle, Ripple formation in weakly turbulent flow, European Physical Journal E 25, 213-221 (2008) (DOI: 10.1140/epje/i2007-10282-4)