UiO Mechanics Seminar Series - Page 2

Mithun Ravisankar is a postdoctoral researcher at the Department of Mathematics, University of Oslo. His research interests lie in experimental studies of bubbles, drops, and granular flows—systems where small details lead to fascinating dynamics.

Susanne Liese is a postdoctoral researcher at University of Augsburg and part of the "mesoscopic physics of life" group. Her research interests lie in wetting phenomena and membrane dynamics within living systems.

Halim Kusumaatmaja is a Professor at the School of Engineering of the University of Edinburgh. His research group is interested in Multiscale Fluid Mechanics, Soft Matter and Biophysics at the interface between Engineering, Physics, Applied Mathematics, Chemistry, and Biology.

Mattia Gazzola is Associate Professor in the Mechanical Science and Engineering Department at the University of Illinois Urbana-Champaign. His work lies at the interface between mechanics, biology, robotics, and computing.

Sunghwan (Sunny) Jung is a Professor in the Department of Biological and Environmental Engineering at Cornell University. His research interest is to investigate various physics and biology problems emerging from the interaction of biological systems with surrounding environments. His group has integrated mathematical modeling, fluid dynamics, and physical & biological experiments to understand animal behaviors and plant physiology for better functions and performance.

Dynamic wetting phenomena are present in many everyday situations where liquids interact with surfaces. Understanding and controlling the dynamics of droplets wetting solid surfaces is important in various applications such as boiling and condensation heat transfer, printing and coating, and microfluidic processes. However, modeling their dynamics is not easy because they are multiscale systems in which the advancing or receding of a nanoscale-thick contact line is linked to the behavior of the entire millimeter-scale droplet. In the Thermal Energy Engineering lab (TEEL) at the University of Tokyo, together with the collaborators, we have been investigating the rapid spreading of droplets on solid surfaces as a model system, where the non-equilibrium nature of energy dissipation at the contact line becomes evident. On a partially wetting solid surface, when capillary forces drive droplet spreading, the advancement of the contact line is governed by viscous forces, inertial forces, or contact-line friction. Through experiments, we demonstrated that the dominant physical mechanisms can be switched by tuning droplet properties, surface microstructure [1], or applied electric field [2]. We modeled and reproduced these experiments using a phenomenological parameter that quantifies the contact-line friction coefficient and mapped a phase diagram of dominant physical factors in a dimensionless parameter space. With the knowledge, we realized anisotropic wetting and droplet transport by taking advantage of the fact that the dynamic behavior of droplets can be controlled through solid surfaces when contact line friction is dominant [3,4]. Furthermore, in our recent experiments on droplet oscillation [5] and droplet sliding [6] on solid surfaces functionalized with various self-assembled monolayers (SAMs), we observed that nanoscale surface chemistry and morphology significantly influence contact-line dynamics. This suggests that contact-line friction originates from nanoscale energy dissipation, though the dissipation mechanism remains unclear, and thus, it calls for collaboration with experts with multiscale discipline.

Uneven solid surfaces are ubiquitous in Nature and can significantly impact fluid flow over them. In this presentation I will discuss two different configurations were the wall unevenness govern fluid transport.
First, I will consider wet ceilings. When liquid is coated on an upside-down surface, it destabilizes to form pendant drops surrounded by a thin film of liquid. I will show that these pendant drops ‘feel’ surface defects even though they have no contact line. The defects induce a gravito-capillary pinning-like force that can be harnessed to control the drop motion.
Second, I will consider hairy pipes. Many biological surfaces are covered with long and elastic filaments or ‘hairs’. I will show that when a fluidic channel is covered with hairs, the hydraulic resistance can become non-linear. This non-linearity is mediated by the hair flexibility and degree of channel obstruction and can be rationalized with a multi-scale model.

In recent years there have been numerous studies on floating films of non-Newtonian fluids due to their application to model the stability of floating ice shelves. Sayag & Worster (2019) showed theoretically that expanding cylinders of shear-thinning fluids can become non-axisymmetrical at the advancing fluid edge, driven by their non-Newtonian rheology. This instability carries over to radially spreading, thinning films of the same material when that fluid floats on a deeper bath of less viscous, but denser, Newtonian fluid.

Experimentally validating these theoretical results has, however, been challenging. Initial experimental exploration using Xanthan gum floating on a dense bath of salt solution shows dramatic non-axisymmetrical patterns. These patterns have their origin in interfacial effects, rather than Sayag & Worster's instability, with the expanding film appearing to fracture under extensional stresses at the fluid edge (Ball et al., 2022).

In this talk, I will present a new set of experiments exploring the instability, in which spreading films of aqueous suspensions of Carbopol and Xanthan gum are floated on a bath of more dense perfluorinated oil. The immiscibility between the bath and floating film prevents the failure of the material under tension and allows comparison against theoretical predictions from a shallow film model (Ball & Balmforth, 2021; Ball & Balmforth 2025).

Multiphase flow in pipes is very complex due to the large number of parameters involved. Differences in phase combinations (e.g. gas-liquid-liquid) and inclination angle can significantly affect the flow regime within the pipe. This in turn can lead to differences in demand on pressure support systems or other facilities. Therefore, accurate estimation of the flow behaviour is crucial for equipment design and operational safety of the production. In this talk, various experimental measurement techniques for monitoring multiphase flow in pipes will be discussed that are currently in use at the Well Flow Loop installation at IFE, Kjeller. In addition, some results from the ongoing research project focusing on flow in steeply inclined pipes will be shown.

Recent advances in experimental techniques have enabled remarkable discoveries and insight into how the dynamics of thin gas/vapour films can profoundly influence the behaviour of liquid droplets: drops impacting solids can “skate on a film of air” [1], so that they can “bounce off walls” [2,3]; reductions in ambient gas pressure can suppress splashing [4] and initiate the merging of colliding droplets [5]; and evaporating droplets can levitate on their own vapour film [7] (the Leidenfrost effect). Despite these advances, the precise physical mechanisms governing these phenomena remain a topic of debate.  A theoretical approach would shed light on these issues, but due to the strongly multiscale nature of these processes brute force computation is infeasible.  Furthermore, when films reach the scale of the mean free path in the gas (i.e. ~100nm) and below, new nanoscale physics appears that renders the classical Navier-Stokes paradigm inaccurate.

In this talk, I will overview our development of efficient computational models for the aforementioned droplet dynamics in the presence of gas nanofilms into which gas-kinetic, van der Waals and/or evaporative effects can be easily incorporated [8,9].  It will be shown that these models can reproduce experimental observations – for example, the threshold between bouncing and wetting for drop impact on a solid is reproduced to within 5%, whilst a model excluding either gas-kinetic or van der Waals effects is ~170% off!  These models will then be exploited to make new experimentally-verifiable predictions, such as how we expect drops to behave in reduced pressure environments.  Finally, I will conclude with some exciting directions for future work.

[1] JM Kolinski et al, Phys. Rev. Lett.  108 (2012), 074503. [2] JM Kolinski et al, EPL.  108 (2014), 24001. [3] J de Ruiter et al, Nature Phys.  11 (2014), 48. [4] L Xu et al, Phys. Rev. Lett. 94 (2005), 184505. [5] J Qian & CK Law, J. Fluid. Mech. 331 (1997), 59.  [6] KL Pan J. Appl. Phys. 103 (2008), 064901. [7] D Quéré, Ann. Rev. Fluid Mech. 45 (2013), 197.  [8] JE Sprittles, Phys. Rev. Lett.  118 (2017), 114502.  [9] MV Chubynsky et al, Phys. Rev. Lett.. 124 (2020), 084501.
Much of this work is based on a recent Annual Review: JE Sprittles, Ann. Rev. Fluid Mech. 56 (2024), 91.

The internal waves in consideration are generated locally in combination by:
1) the North Atlantic Current (NAC) that flows along the Norwegian shelf (average speed 0.3 m/s),
2) the local tide (dominant 24 hr mode),
3) the particular shelf geometry,
4) the seasonal pycnocline (at 20 m depth). The typical amplitude is 50 m. The local depth where they exist is 100 m (approx.).
The local internal wave speed of 0.4 m/s is comparable to the current speed.

The wave generation process occurs in a sequence of:
A) locally large depressions of the internal tide occurring at the lee side of two shallow ridges,  
B) the depressions are orthogonal to the current,
C) the depressions move slowly against the current, and
D) eventually fission into nonlinear-dispersive wave groups,
E) they propagate upstream against the current,
F) they are recorded on satellite images and is visible by eye.
 

In a recent RCN-funded project (ECOPULSE) 2020-2023 including research groups at UiO, the Norwegian Meteorological Institute and the Institute of Marine Research in Bergen, field experiments of the mother wave in points A) and C) were undertaken during July-September 2021 and 2022. The measurements show that the waves break and mix in the entire water column by convective breaking in the upper part of the water column, shear instability within the pycnocline, and by a variant of a shear instability near the bottom. The field measurements document the local wave properties and the initial phase of the fission process. The Vesterålen-Lofoten area is of immense political interest because of a) its traditional fisheries and breeding ground for the North Atlantic and Barents Sea Cod and other species, where also a cold water coral reef is located right where these waves are generated.  The waves do also extend across NORDLAND VII, an oil / gas field which may be considered for exploitation. Preliminary results will be presented.

The efficient redistribution and control of flow is essential in many biological and engineered systems, from our cardiovascular system to plants and soft robotics. Inspired by nature, microfluidic devices with passive valves have been developed to perform a variety of tasks, from cell manipulation to fluid mixing and reaction control, giving rise to the field of soft hydraulics. In this talk, we present two prototypes of passive valves: one harnessing buckling in slender beams and another leveraging snapping in spherical shells, where fluid-induced elastic instabilities are tamed to achieve function. Through a combination of precision desktop-scale experiments, numerical simulations, and theoretical analysis, we rationalize these behaviors and provide design rules based on scaling regimes of material, geometric, and fluid parameters.

Earthquake, landslide, and glacier instabilities are multi-scale processes that cannot be predicted. When a rock is subjected to large stress, it can develop fractures that grow and connect to form a network. This network of fractures spans the entire rock right before it fails catastrophically. Despite extensive research on rock damage, researchers are still far from being able to predict when a rock will fail. Here, I will show how to use a multi-view convolutional neural network model to identify characteristics of a failing rock. By training a neural network model on images of rock samples exposed to different levels of stress, one can predict how close a rock is to failure. The neural network model outperforms traditional estimates based on fracture density. More importantly, the model provides fundamental insight that may offer precursory information on impending material failure. For instance, according to the model, the angle of the fracture plane relative to the principal loading direction becomes a key factor contributing to failure.

Swimming microorganisms and artificial microswimmers use environmental cues to influence their dynamics, helping them to improve transport, reach specific flow regions, or avoid harmful conditions. Although plankton navigate turbulent flows in nature, it is poorly understood how they do so efficiently. By combining Reinforcement Learning with analytical perturbation theory, we investigate how microswimmers can sense environmental signals and adjust a few control parameters to navigate turbulent flows. This approach helps identify effective navigation strategies for various tasks. We illustrate this with two key tasks: avoiding high-strain regions and navigating against gravity, both essential for plankton in the ocean.