08:45  09:00

Frank Jülicher (MPIPKS) and scientific coordinators
Opening


Chair: Paul Steen

09:00  09:30

Daniel Bonn
(University of Amsterdam)
Wetting and lubrication
We investigate the transition between different regimes of lubrication, and present a new method to directly observe the thickness of the nanometric lubrication films with a sensitivity of a single molecular layer between rough surfaces. This allows for measurement of the structure of the lubrication film in three dimensions and with unprecedented resolution. We use a novel definition of the specific film thickness successfully captures the transition from full elastohydrodynamic lubrication to mixed and boundary lubrication.

09:30  10:00

Stephen Wilson
(University of Strathclyde)
Evaporating droplets
Determining the lifetime of an evaporating sessile droplet on a solid substrate is an important part of understanding many industrial processes, such as inkjet printing, coating, and spray cooling, as well as drug delivery systems and chemical spill containment. Consequently, in recent years there has been a rapid growth of experimental and theoretical research into droplet evaporation. Previous authors [1] have shown that the instantaneous evaporation rate of a droplet depends on the thermal conductivity of the substrate. In the present work we use a combination of analytical and numerical methods to study the effect of the thermal conductivity of the substrate on the evolution, and hence the lifetime, of an evaporating droplet. In the limit of a thin droplet on a thin substrate we obtain analytical expressions for the lifetimes of droplets evaporating in various modes of evaporation [3]. In the general case of a nonthin droplet we use numerical methods to calculate the lifetimes of droplets evaporating in various modes of evaporation on substrates of various thermal conductivities. In general, droplets on less conductive substrates are found to have longer lifetimes than those on more conductive ones.
References
[1] Dunn G. J., Wilson S. K., Duffy, B. R., David, S. and Sefiane K. "The strong influence of substrate conductivity on droplet evaporation" Journal of Fluid Mechanics, 2009, 623, 329351.
[2] Stauber J. M., Wilson S. K., Duffy, B. R. and Sefiane K. "On the lifetimes of evaporating droplets" Journal of Fluid Mechanics, 2014, 744, R2.
[3] Schofield F. G. H., Wilson S. K., Pritchard, D. and Sefiane K. "The lifetimes of evaporating sessile droplets are significantly extended by strong thermal effects" Journal of Fluid Mechanics, 2018, 851, 231244.

10:00  10:30

Serafim Kalliadasis
(Imperial College London)
The resolution of the moving contact line problem
The resolution of the moving contact line
The moving contact line problem occurs when modelling one fluid replacing another as it moves along a solid surface, a situation widespread throughout industry and nature. Classically, the noslip boundary condition at the solid substrate, a zerothickness interface between the fluids, and motion at the threephase contact line are incompatible  leading to the wellknown shearstress singularity. At the heart of the problem is its multiscale nature: a nanoscale region close to the solid boundary where the continuum hypothesis breaks down, must be resolved before phenomenological macroscale parameters such as contact line friction and slip, often adopted to alleviate the singularity [1], can be obtained.
Here we will review recent progress made by our group to rigorously analyse the moving contact line problem and related physics from the nano to macroscopic lengthscales. Specifically, to capture nanoscale properties very close to the contact line and to establish a link to the macroscale behaviour, we employ elements from the statistical mechanics of classical fluids, namely densityfunctional theory (DFT) [2,3]. We formulate a new and general dynamic DFT (DDFT) [4]that carefully and systematically accounts for the fundamental elements of any classical fluid and soft matter system, a crucial step towards the accurate and predictive modelling of physically relevant systems. In a certain limit, our DDFT reduces to a nonlocal NavierStokeslike equation [5]: an inherently multiscale model, bridging the micro to the macroscale, and retaining the relevant fundamental microscopic information (fluid temperature, fluidfluid and wallfluid interactions) at the macroscopic level.
Work analysing the contact line in both equilibrium and dynamics will be presented [6,7]. The new model allows us to benchmark existing phenomenological models and reproduce some of their key ingredients. But its multiscale nature also allows us to unravel the underlying physics of moving contact lines, not possible with any of the previous approaches, and indeed show that the physics is much more intricate than the previous models suggest. For instance, a key property captured by our theory is the fluid layering at the wallfluid interface, amplified as the contact angle decreases. But also the existence of compressive interfacial regions on the vapor side of the vaporliquid interface and a large shear region close to the wall in which effective slip can be generated. We demonstrate that the stratified fluid structure in the vicinity of the wall has a large effect on the compression and shearing properties of the fluid and determines the width of the shear region on the wall. We also scrutinize the effect of stratification on contact line friction and the dependence of the latter on the imposed temperature of the fluid and motion orientation [8].
Selected references
[1] D.N. Sibley, A. Nold and S. Kalliadasis 2015 "The asymptotics of the moving contact line: cracking an old nut," J. Fluid Mech. 764, 445462.
[2] P. Yatsyshin, N. Savva and S. Kalliadasis 2015 "Wetting of prototypical one and twodimensional systems: Thermodynamics and density functional theory," J. Chem. Phys. 142, Art. No. 034708.
[3] P. Yatsyshin, A.O. Parry and S. Kalliadasis 2016 "Complete prewetting," J. Phys.: Condens. Matter 28, Art. No. 275001.
[4] B.D. Goddard, A. Nold, N. Savva, G.A. Pavliotis and S. Kalliadasis 2012 "General dynamical density functional theory for classical fluids," Phys. Rev. Lett. 109, Art. No. 120603.
[5] B.D. Goddard, A. Nold, N. Savva, P. Yatsyshin and S. Kalliadasis 2013 "Unification of dynamic density functional theory for colloidal fluids to include inertia and hydrodynamic interactions: derivation and numerical experiments," J. Phys.: Condens. Matter 25, Art. No. 035101.
[6] A. Nold, D.N. Sibley, B.D. Goddard and S. Kalliadasis 2014 "Fluid structure in the immediate vicinity of an equilibrium threephase contact line and assessment of disjoining pressure models using density functional theory," Phys. Fluids 26, Art. No. 072001.
[7] A. Nold, D.N. Sibley, B.D. Goddard and S. Kalliadasis 2015 "Nanoscale fluid structure of liquidsolidvapor contact lines for a wide range of contact angles," Math. Model. Nat. Phenom. 10, 111125.
[8] A. Nold, PhD Thesis, Imperial College London (2016).

10:30  11:00

coffee break


Chair: Daniel Bonn

11:00  11:30

Günter Auernhammer
(Leibniz Institute of Polymer Research Dresden)
Influence of substrate and surface rheology on wetting
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 quasistatic 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): 15441547.
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.

11:30  12:00

Vladimir Ajaev
(Southern Methodist University)
Evaporation and interface dynamics in microregion on heated substrate of nonuniform wettability
Modeling of evaporation in the microregion separating an adsorbed film and a macroscopic meniscus is important for a number of applications such as pool boiling and micro heat pipes. We develop a model of a moving microregion incorporating the effects of evaporation, viscous flow, surface tension, and twocomponent disjoining pressure and apply it to study contact line motion over heated surfaces with wettability defects or patterns. Substantial heat transfer enhancement is found when a receding microregion passes over the portion of the substrate with higher wettability than the surrounding areas. The effect is explained in physical terms by widening of the part of the microregion with low thermal resistance. To achieve sustained heat transfer enhancement, we then consider a configuration in which the substrate is patterned by an array of highwettability stripes and investigate the evaporative flux as a function of the geometry of the pattern. The effects of Marangoni stress at the interface are also studied and found to result in slight reduction of the average evaporative flux, as the tangential stress at the interface reduces the liquid supply into the micro region. Joint work with E. Gatapova and O. Kabov.

12:00  12:30

Roland Knorr
(Max Planck Institute of Colloids and Interfaces)
Intracellular wetting regulates autophagic degradation of fluid cargoes
Phase separation generates functional, subcellular condensates with fluidlike properties. The mechanisms that degrade fluid phases in cells, however, are not fully understood. Clearance of condensates involves autophagy, a conserved pathway in which membrane sheets isolate portions of the cytosol to form autophagosomes, which then fuse with lysosomes for cargo breakdown. Here, we studied the assembly of autophagic membranes at fluid interfaces in both living and synthetic cells. A minimal physical model shows that interfacial tension determines whether isolation membranes will sequester the condensate entirely, or will prematurely close and degrade the condensate in a piecemeal fashion. In the latter case, curvatureinducing proteins confer a regulatory switch that associates isolation membranes with condensates, however, without degrading them. Thus, condensates persist and serve as a platform to assemble cytosoldegrading autophagosomes repeatedly. Our findings suggest that the autophagic breakdown of fluids  fluidophagy  is a wetting phenomenon, which is controlled by elastocapillary feedback between the phaseseparated condensate and the isolating membrane.

12:30  13:30

lunch

13:30  14:00

discussion


Chair: Ilia Roisman

14:00  14:30

Jens Eggers
(University of Bristol)
Dynamic drying transition via freesurface cusps
We study air entrainment by a solid plate plunging into a viscous liquid, theoretically
and numerically. At dimensionless speeds $Ca = U\eta/\gamma$ of order unity, a nearcusp
forms due to the presence of a moving contact line. The radius of curvature of the
cusp’s tip scales with the slip length multiplied by an exponential of $−Ca$. The
pressure from the air flow drawn inside the cusp leads to a bifurcation, at which air
is entrained, i.e. there is ‘wetting failure'. We develop an analytical theory of the
threshold to air entrainment, which predicts the critical capillary number to depend
logarithmically on the viscosity ratio, with corrections coming from the slip in the
gas phase.

14:30  15:00

Mark Wilson
(University of Leeds)
Impact of small liquid droplets on topographies of commensurate size
The use of inkjet printing to manufacture printed electronics has been receiving increasing attention due to the technique's potential for rolltoroll manufacturing, lower cost and flexibility. As the droplets used for printing become smaller O(10um), substrate topographical features (designed or due to minor variations) can affect the dynamics and equilibrium morphologies of the printed liquid. Here, the interaction of droplets with a surface featuring an open microchannel are explored. Such a feature could represent a designed structure and allow us to investigate the effect of its dimensions on the printed morphology. A 3D GPUenabled multiphase Lattice Boltzmann Method is used. The GPU speedup enables running parametric studies in affordable simulation time. This implementation also captures partial wetting and contact angle hysteresis. The latter is significant as stable continuous printed lines form only if the receding contact angle is very low or zero. The model is validated using experimental data from the literature.
The effect on dynamics and equilibrium morphologies of a single droplet as the depth and width of the feature change is investigated using the developed simulation methodology. Varying the width and depth of the microchannel relative to the droplet size reveals six different morphologies varying from complete imbibition into the microchannel to almost just a spherical cap. In addition, a series of droplets that form a continuous printed line are simulated to ascertain what depths and widths of the feature cause breaks in continuity of the line.

15:00  15:30

Ehud Yariv
(Israel Institute of Technology)
Speed of rolling droplets
We analyze the nearrolling motion of twodimensional nonwetting drops down a gently inclined plane. Inspired by the scaling analysis of Mahadevan & Pomeau, we focus upon the limit of small Bond numbers, $B\ll1$, where the drop shape is nearly circular {and the internal flow is approximately a rigidbody rotation except close to the flat spot at the base of the drop.} Our analysis reveals that the leadingorder dissipation is contributed by both the flow in the flatspot region and the correction to rigidbody rotation in the remaining liquid domain. The resulting leadingorder approximation for the drop velocity $\mathcal{U}$ is given by
$\mu\, \mathcal{U}/{\gamma} \sim {\alpha}/{2B\ln\frac{1}{B}}$,
wherein $\mu$ is the liquid viscosity, $\gamma$ the interfacial tension and $\alpha$ the inclination angle.

15:30  16:00

discussion

16:00  16:30

coffee break


Chair NAWET19 Colloquium: Steffen Rulands (MPIPKS)

16:30  17:30

nawet19 colloquium: David Quéré
(ESPCI & Ecole polytechnique)
Special dynamics of water pearls
A few tricks (hydrophobic texture, Leidenfrost state, etc.) allow us to keep water drops with a spherical shape, which induces spectacular properties. We describe a few of them. We also challenge the water mobility in the cases where it is expected to be impeded  for instance by reducing the drop size or by increasing the water temperature.

17:30  18:30

discussion

18:30  19:30

dinner

19:30  21:00

discussion
