06.09.2019

David Marpaung
_{Universität Twente}

Stimulated Brillouin scattering (SBS), whereby light interacts coherently with acoustic phonons is a powerful and flexible mechanism for the control of light. Until recently, this effect was limited to a long length of optical fiber. Recent advances in nanofabrication have opened new possibilities for harnessing Brillouin interaction in nanoscale optical circuits. In this talk, I will review the principles of Brillouin scattering and discuss the ways to harness SBS in a photonic chip, leading to exciting new technologies including tunable microwave filters and novel on-chip Brillouin lasers. 

Modeling and computing thin films on thermally conductive substrates

Lou Kondic
_{New Jersey Institute of Technology}

The talk will focus on modeling evolution of thin fluid films exposed to external heat source on a thermally conductive substrate. The particular case that will be used as a motivation involves evolving metal films of nanoscale thickness exposed to external laser heating; however the mathematical models and supporting computations are more general and could apply to a variety of materials and heating mechanisms. One challenge in considering films on thermally conductive substrates involves coming up with accurate models for evolution of thermal energy that could be efficiently coupled with the fluid mechanical evolution of the film itself. The talk will also touch on some new and not completely understood results including the influence of thermal dependence of material parameters on the film evolution as well as the possibility of oscillatory film instabilities, among others.

Multi-component droplet dynamics: Rayleigh vs. Marangoni

Dr. Christian Diddens
_{University of Twente}

In multi-component liquid systems, differences in volatilities or solubilities can induce compositional gradients, which in turn can drive Marangoni flow due to a difference in surface tension and Rayleigh convection as a consequence of a composition-dependent mass density. One famous example llustrating the interplay between Marangoni flow and gravity is known as "tears of wine". Due to preferential evaporation of ethanol from a glass of wine, the liquid is pulled up along the glass, whereafter it falls back due to gravity, resulting in the well-known curtain-like structures. In this talk, the competition of gravity and the Marangoni effect is discussed on the basis of two different systems, namely the evaporation of a binary droplet on a substrate and the suprising behavior of an oil droplet submerged in a vertical ethanol-water gradient. Both systems are investigated in detail by experimental and numerical means and the relevant regions are revealed in the parameter space in terms of Rayleigh and Marangoni numbers.

Dr. Axel Rühl
_{Leibniz Universität Hannover, Institut für Quantenoptik}

The advent of the optical frequency comb in the late 1990’s lead to a remarkable progress in various fields of fundamental and applied science. Initially invented for frequency metrology, such frequency combs enable new approaches to spectroscopy, of particular relevance to molecules. The performance of existing spectroscopy techniques can be dramatically enhanced enabling fast and accurate measurements over broad spectral ranges. The direct self-calibration of the frequency scale possible within the accuracy of an atomic clock and the negligible contribution of the instrumental line-shape can enable the determination of all spectral parameters with high accuracy for stringent comparisons with theories in molecular physics.
This talk discusses the measurement of line shape parameters of the fundamental carbon monoxide vibrational transitions around 4.6 μm with a custom Fourier-transform spectrometer using a mid-infrared frequency comb based on difference frequency generation as the light source. This approach allows measuring instrumental line-shape-free broadband molecular spectra by precisely matching the maximum delay range of the spectrometer to the comb line spacing resulting in undistorted high-resolution spectra with a frequency scale accuracy only limited by the frequency comb. The talk will also discuss phase- and amplitude noise optimization and stabilization of combs based on difference frequency generation as a mandatory prerequisite for precision spectroscopy experiments.

Achtung: Abweichung vom gewohnten Dienstagstermin

Prof. Dr. Günter Steinmeyer
_{Max‐Born‐Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Berlin}

Rogue waves are waves of extreme amplitude, which grossly exceed expectations from Gaussian statistics. They are said to come out of nowhere and disappear without a trace, and they may cause fatal damage to ships and other maritime structures. Long considered sailor’s yarn, the actual observation of a rogue wave at the Draupner platform on New Year’s Day of 1995 greatly changed our view on these extreme waves. A decade later, starting with a report of rogue solitons in a nonlinear optical fiber, similar extreme‐value statistics were also reported for numerous optical systems. While ocean rogue waves are rare events, optical systems enable the observation of hundreds or thousands of such waves in a few seconds’ time. With such an extensive data base, one can now run a number of statistical tests, for example, on the predictability of these extreme waves. And in fact, fiber‐optical rogue waves truly come out of nowhere, yet their ocean counterparts bear a certain, yet rather small amount of predictability, i.e., prediction of the individual event appears to be impractical but not totally impossible.

Nevertheless, extending our predictability analysis to phase space reconstruction, we find that the “rogueness” of the ocean seems to be variable. This feat may allow for an estimate on the likelihood of the ocean giving birth to these monstrous waves. That said, rogue waves may become predictable similar meteorological forecasts of thunderstorms, tornados, and other atmospheric extreme events.

Flexible and liquid interfaces: The influence of substrate and surface rheology on wetting

Dr. Günter Auernhammer
_{Leibniz-Institut für Polymerforschung Dresden}

Drops sliding down an inclined wall can have very different shapes, depending on their composition. This is a simple everyday example of the complex interplay between drop shape, substrate properties and dynamics of wetting. In static situations, liquid surfaces are characterized by a surface tension that depends on thermodynamic quantities like temperature and composition of the liquid. For dynamic situations, the surfaces tension becomes additionally a function of, e.g., the deformation rate of the surface. A tutorial example is sitting drop on a deformable immiscible substrate. I will discuss changes in drop shape, condensation, and drop motion. The surface tension exerts a vertical force to the substrate. For soft enough substrates (low elastic modulus of the substrate) this leads to a deformation of the substrate under the drop. Not only quasi-static properties, like nucleation barrier [1] or drop shape are influences, also the dynamics of the drop (sliding) is strongly changed [2]. Here, the viscosity or viscoelasticity of the substrate influences the drop motion. Liquid surfaces have rheological properties that are independent of the bulk properties. The surface alone can exhibit elastic or viscoelastic restoring forces against shear or extensional deformation. Even at identical static surfaces properties, the dynamics behavior of drops strongly differ due to surface rheology of the liquid. I will give some examples on this and discuss, e.g., dynamic receding contact angles [3].

References

1. Sokuler, M., G. K. Auernhammer, M. Roth, C. Liu, E. Bonacurrso and H.-J. Butt (2009). "The Softer the Better: Fast Condensation on Soft Surfaces." Langmuir 26(3): 1544-1547.

2. Karpitschka, S., S. Das, M. van Gorcum, H. Perrin, B. Andreotti and J. H. Snoeijer (2015). "Droplets move over viscoelastic substrates by surfing a ridge." Nat Commun 6.

3. Henrich, F., D. Fell, D. Truszkowska, M. Weirich, M. Anyfantakis, T.-H. Nguyen, M. Wagner, G. K. Auernhammer and H.-J. Butt (2016). "Influence of surfactants in forced dynamic dewetting." Soft Matter 12: 7782 - 7791.

Prof. Philipp Maass
_{Universität Osnabrück}

Models of driven stochastic particle transport in one dimension have been applied to
describe such diverse phenomena as biopolymerization, molecular motor motion along
filaments, flow of molecules through nanopores, ion conduction through membrane
channels, electron transport along molecular wires, or vehicular traffic. A simple lattice
model, the asymmetric simple exclusion process (ASEP) appears as a basic building
block in the theoretical description of these driven diffusion systems and has developed
into one of the standard models for investigating nonequilibrium steady states. After a
short introduction to the physics of the ASEP and some model variants with the focus
on nonequilibrium phase transitions [1,2], we address the question whether
corresponding phenomena will occur in driven Brownian motion, making them
amenable to experimental investigations utilizing advanced techniques of microfluidics
or optical/magnetic micromanipulation. Specifically, we introduce the Brownian
asymmetric simple exclusion process (BASEP) with overdamped Brownian dynamics
and a setup resembling that of the ASEP on a lattice [3-5]. In this BASEP, particles of
size s with hardcore interaction are driven by a constant drag force through a periodic
potential with wavelength l and an amplitude much larger than the thermal energy.
We show that the character of the non-equilibrium steady states in the BASEP is
strikingly different from that in the ASEP. Compared with a system of non-interacting
particles, the current is enhanced for small s/l ratios due to a barrier reduction effect
arising from multi-occupation of potential wells. Larger s/l ratios lead to a suppression
of the current because of blocking effects. Surprisingly, an exchange-symmetry effect
causes the current-density relation to be identical to that of non-interacting particles for
commensurable lengths s=nl, n=1,2... A behavior similar as for the ASEP is obtained
only in a limited parameter regime. The rich behavior of the current-density relation
leads to phase diagrams of nonequilibrium steady states with up to five different
phases. The structure of these phase diagrams changes with varying s/l ratio.

[1] M. Dierl, P. Maass, and M. Einax, Phys. Rev. Lett. 108, 060603 (2012).
[2] M. Dierl, M. Einax, and P. Maass, Phys. Rev. E 87, 062126 (2013).
[3] D. Lips, A. Ryabov, and P. Maass, Phys. Rev. Lett. 121, 160601 (2018).
[4] A. Ryabov, D. Lips, and P. Maass, J. Phys. Chem. C 123, 5714 (2019).
[5] D. Lips, A. Ryabov, and P. Maass, Phys. Rev. E 100, 052121 (2019).

Pattern-formation and Critical Behaviour in Dipolar Bose-Einstein Condensates

Dr. Fabian Maucher
_{Department of Physics and Astronomy, Aarhus University}

The formation of patterns with long-range order continues to fascinate scientists from a broad range of natural sciences [1]. In this talk I will discuss pattern-formation in dipolar Bose-Einstein condensates. In this system the possibility of obtaining quantum states with self-organized long-ranged ordering can be facilitated by quantum fluctuations which suppress collapse and pave the way for supersolids in this system [2]. Here, supersolidity refers to a state of matter which displays long-range ordering whilst maintaining a large superfluid fraction. I will present recent results which focus on the critical behaviour of the superfluid-supersolid phase-transition [3] and the crucial role quantum fluctuations can play for the latter. We find that quantum fluctuations can alter the order of the phase transition from first- to second-order. Furthermore, apart from the usual triangular lattice of density droplets, quantum fluctuations can give rise to a novel quantum state whose density distribution displays a honeycomb structure.

[1] A. M. Turing, Philos. Trans. Royal Soc. B 23 237 (1952).
[2] H. Kadau, M. Schmitt, M. Wenzel, C. Wink, T. Maier, I. Ferrier-Barbut, T. Pfau, Nature 530 194 (2016).
[3] Y. Zhang, F.M., T. Pohl, Phys. Rev. Lett. 123 015301 (2019).