Lifetime of vertical giant soap films: role of the relative humidity and film dimensions.

We consider the lifetime of rectangular vertical soap films and we explore the influence of relative humidity and both dimensions on the stability of large soap films, reaching heights of up to 1.2 m. Using an automated rupture detection system, we achieve a robust statistical measurement of their lifetimes and we also measure the film thinning dynamics. We demonstrate that drainage has a negligible impact on the film stability as opposed to evaporation. To do so, we compare the measured lifetimes with predictions from the Boulogne & Dollet model, originally designed to describe the convective evaporation of hydrogels. Interestingly, we show that this model can predict a maximum film lifetime for all sizes.

Corrosion-driven droplet wetting on iron nanolayers.

The classical Evans' drop describes a drop of aqueous salt solution, placed on a bulk metal surface where it displays a corrosion pit that grows over time producing further oxide deposits from the metal dissolution. We focus here on the corrosion-induced droplet spreading using iron nanolayers whose semi-transparency allowed us to monitor both iron corrosion propagation and electrolyte droplet behavior by simple optical means. We thus observed that pits grow under the droplet and merge into a corrosion front. This front reached the triple contact line and drove a non radial spreading, until it propagated outside the immobile droplet. Such chemically-active wetting is only observed in the presence of a conductive substrate that provides strong adhesion of the iron nanofilm to the substrate. By revisiting the classic Evan's drop experiment on thick iron film, a weaker corrosion-driven droplet spreading is also identified. These results require further investigations, but they clearly open up new perspectives on substrate wetting by corrosion-like electrochemical reactions at the nanometer scale.

Evaporation-Induced Temperature Gradient in a Foam Column.

Various parameters affect foam stability: surface and bulk rheology of the solution, gravitational drainage, mechanical vibrations, bubble gas composition, and also evaporation. Evaporation is often considered through the prism of liquid loss but also induces a cooling effect due to the enthalpy of vaporization. In this study, we combine a theoretical and experimental approach to explore the temperature field in a foam column evaporating from the top. We show that a measurable temperature profile exists in this geometry, with temperatures at the interface lower than the environmental temperature by a few degrees. We demonstrate that the temperature profile is the result of a balance between the enthalpy of vaporization and heat fluxes originating from the thermal conduction of foam and air and thermal radiation. For small foam thicknesses compared to the radius, we found that the temperature gradient is established over the foam thickness, while for large aspect ratios, the gradient spans over a length scale comparable to the tube radius.

Giant slip length at a supercooled liquid-solid interface.

The effect of temperature on friction and slip at the liquid-solid interface has attracted attention over the last 20 years, both numerically and experimentally. However, the role of temperature on slip close to the glass transition has been less explored. Here we use molecular dynamics to simulate a bidisperse atomic fluid, which can remain liquid below its melting point (supercooled state), to study the effect of temperature on friction and slip length between the liquid and a smooth apolar wall in a broad range of temperatures. At high temperatures, an Arrhenius law fits well the temperature dependence of viscosity, friction, and slip length. In contrast, when the fluid is supercooled, the viscosity becomes super-Arrhenian, while interfacial friction can remain Arrhenian or even drastically decrease when lowering the temperature, resulting in a massive increase of the slip length. We rationalize the observed superlubricity by the surface crystallization of the fluid, and the incommensurability between the structures of the fluid interfacial layer and of the wall. This study calls for experimental investigation of the slip length of supercooled liquids on low surface energy solids.

Self-organization and dewetting kinetics in sub-10 nm diblock copolymer line/space lithography.

In this work, we investigated the self-assembly of a lamellar block copolymer (BCP) under different wetting conditions. We explored the influence of the chemical composition of under-layers and top-coats on the thin film stability, self-assembly kinetics and BCP domain orientation. Three different chemistries were chosen for these surface affinity modifiers and their composition was tuned in order to provide either neutral wetting (i.e. an out-of-plane lamellar structure), or affine wetting conditions (i.e. an in-plane lamellar structure) with respect to a sub-10 nm PS-b-PDMSB lamellar system. Using such controlled wetting configurations, the competition between the dewetting of the BCP layer and the self-organization kinetics was explored. We also evaluated the spreading parameter of the BCP films with respect to the configurations of surface-energy modifiers and demonstrated that BCP layers are intrinsically unstable to dewetting in a neutral configuration. Finally, the dewetting mechanisms were evaluated with respect to the different wetting configurations and we clearly observed that the rigidity of the top-coat is a key factor to delay BCP film instability.

Elastohydrodynamic Relaxation of Soft and Deformable Microchannels.

Hydrodynamic flows in compliant channels are of great interest in physiology and microfluidics. In these situations, elastohydrodynamic coupling leads to (i) a nonlinear pressure-vs-flow-rate relation, strongly affecting the hydraulic resistance; and (ii), because of the compliance-enabled volume storage, a finite relaxation time under a stepwise change in pressure. This latter effect remains relatively unexplored, even while the timescale can vary over a decade in typical situations. In this study we provide time-resolved measurements of the relaxation dynamics for thin and soft, rectangular microfluidic channels. We describe our data using a perturbative lubrication approximation of the Stokes equation coupled to linear elasticity, while taking into account the effect of compliance and resistance of the entrance. The modeling allows us to completely describe all of the experimental results. Our Letter is relevant for any microfluidic scenario wherein a time-dependent driving is applied and provides a first step in the dynamical description of compliant channel networks.

An optimized recipe for making giant bubbles.

Large bubbles are primarily used in physics laboratories to study 2D turbulence, surface wavers, and fundamental properties of soap systems. Outside of physics, blowing large bubbles is also a part of many performances and shows. Both the scientific and artistic communities usually want to get reasonably stable foam films. The purpose of this article is to propose the main physical ingredients needed for an effective recipe to make stable films and bubbles. We propose controlled experiments to measure both the ease of generating a bubble, and its stability in different stabilizing solutions, which we choose by adding the ingredients contained in a bubble artist's recipe one by one. Our main findings are that (i) the surfactant concentration must not be too high, (ii) the solution must contain long flexible polymer chains to allow for easy bubble generation and (iii) the addition of glycerol provides improved bubble stability by preventing evaporation. Finally, we propose an efficient recipe, which takes into account these considerations.

The impact of physical-chemistry on film thinning in surface bubbles.

In this paper, we investigate the thinning dynamics of evaporating surfactant-stabilised surface bubbles by considering the role of physical-chemistry of solutions used in a liquid bath. We study the impact of the surfactant concentration below and above the cmc (critical micelle concentration) and the role of ambient humidity. First, in a humidity-saturated atmosphere, we show that if the initial thickness depends on the surfactant concentration and is limited by the surface elasticity, the drainage dynamics are very well described from the capillary and gravity contributions. These dynamics are independent of the surfactant concentration. Second, our study reveals that the physical-chemistry impacts the thinning dynamics through evaporation. We include in the model the additional contribution due to evaporation, which shows a good description of the experimental data below the cmc. Above the cmc, although this model is unsatisfactory at short times, the dynamics at long times is correctly rendered and we establish that the increase of the surfactant concentration decreases the impact of evaporation. Finally, the addition of a hygroscopic compound, glycerol, can be also rationalized by our model. We demonstrate that glycerol decreases the bubble thinning rate at ambient humidity, thus increasing their stability.

Measurement of the Temperature Decrease in Evaporating Soap Films.

Recent advances have demonstrated that evaporation can play a significant role on soap film stability, which is a key concern in many industrial areas but also for children playing with bubbles. Thus, evaporation leads to a film thinning but also to a film cooling, which has been overlooked for soapy objects. Here, we study the temperature variation of an evaporating soap film for different values of relative humidity and glycerol concentrations. We evidence that the temperature of soap films can decrease after their creation up to 8 °C. We propose a model describing the temperature drop of soap films after their formation that is in quantitative agreement with our experiments. We emphasize that this cooling effect is significant and must be carefully considered in future studies on the dynamics of soap films.

Nonlinear amplification of adhesion forces in interleaved books.

It is nearly impossible to separate two interleaved phonebooks by pulling their spines. The very slight force exerted by the outer sheets of the assembly is amplified as the exponential of the square of the number of sheets, meaning that even a small number of sheets can create a highly resistant system. We present a systematic and detailed study of the influences of the normal external force and the geometrical parameters of the booklets on the assembly strength. We conclude that the paper-paper adhesion force between the two outer sheets, on the order of a few [Formula: see text],  is the one amplified by the interleaved-book system. The two-phonebook experiment-which has attracted the attention of students and the non-scientific public all around the world as an outstanding demonstration of the strength of friction-appears to also be a spectacular macroscopic manifestation of the microscopic coupling of friction and adhesion.

A new setup for giant soap films characterization.

Artists, using empirical knowledge, manage to generate and play with giant soap films and bubbles. Until now, scientific studies of soap films generated at a controlled velocity and without any feeding from the top, studied films of a few square centimeters. The present work aims to present a new setup to generate and characterize giant soap films (2 m [Formula: see text] 0.7 m). Our setup is enclosed in a humidity-controlled box of 2.2 m high, 1 m long, and 0.75 m large. Soap films are entrained by a fishing line withdrawn out of a bubbling solution at various velocities. We measure the maximum height of the generated soap films, as well as their lifetime, thanks to automatic detection. This is allowed by light-sensitive resistors collecting the light reflected on the soap films and ensures robust statistical measurements. In the meantime, thickness measurements are performed with a UV-VIS-spectrometer, allowing us to map the soap film's thickness over time.

Near-surface rheology and hydrodynamic boundary condition of semi-dilute polymer solutions.

Understanding confined flows of complex fluids requires simultaneous access to the mechanical behaviour of the liquid and the boundary condition at the interfaces. Here, we use evanescent wave microscopy to investigate near-surface flows of semi-dilute, unentangled polyacrylamide solutions. By using both neutral and anionic polymers, we show that monomer charge plays a key role in confined polymer dynamics. For solutions in contact with glass, the neutral polymers display chain-sized adsorbed layers, while a shear-rate-dependent apparent slip length is observed for anionic polymer solutions. The slip lengths measured at all concentrations collapse onto a master curve when scaled using a simple two-layer depletion model with non-Newtonian viscosity. A transition from an apparent slip boundary condition to a chain-sized adsorption layer is moreover highlighted by screening the charge with additional salt in the anionic polymer solutions. We anticipate that our study will be a starting point for more complex studies relating the polymer dynamics at interfaces to their chemical and physical composition.

Roughness-Induced Friction on Liquid Foams.

Complex liquids flow is known to be drastically affected by the roughness condition at the interfaces. We combined stresses measurements and observations of the flow during the motion of different rough surfaces in dry liquid foams. We visually show that three distinct friction regimes exist: slippage, stick-slip motion, and anchored soap films. Our stress measurements are validated for slippage and anchored regimes based on existing models, and we propose a leverage rule to describe the stresses during the stick-slip regime. We find that the occurrence of the stick-slip or anchored regimes is controlled by the roughness factor, defined as the ratio between the size of the surface asperities and the radius of curvature of the Plateau borders.

Using ultrasound to characterize the tongue-food interface: An in vitro study examining the impact of surface roughness and lubrication.

We measured the apparent reflection coefficient of a 1-MHz ultrasound compressional wave at the interface between rough and lubricated tongue mimicking surfaces and various food gels, composed of agar or gelatin. For the smoothest mimicking surface, when a lubricating layer was present, the apparent reflection coefficient was fairly similar for the different food gels (33.6% on average). The apparent reflection coefficient was significantly larger in the following situations: (i) tongue asperities were high and dense; (ii) lubrication levels were low; and (iii) gels were less rigid (range for the different gels-45.9-84.3%). The apparent reflection coefficient conveys the ability of food gels to mold themselves to surface asperities or to form a coupling film of liquid at the interface. This study demonstrates that ultrasound methods can and should be used to explore the physical phenomena that underlie the texture perceptions resulting from tongue-palate interactions.

Viscoelasticity-Induced Onset of Slip at the Wall for Polymer Fluids.

The progressive onset of slip at the wall, which corresponds to a slip length increasing with the solicitation time before reaching a plateau, has been investigated for model viscoelastic polymer solutions, allowing one to vary the longest relaxation time while keeping constant solid-fluid interactions. A hydrodynamic model based on a Maxwell fluid and the classical Navier's hypothesis of a linear response for the friction stress at the interface fully accounts for the data. In the limit of the linear viscoelasticity of the fluid, we could postulate a Newtonian response for the interfacial friction coefficient, reflecting the local character of solid-liquid friction mechanisms. Deviations between the experiments and our model are observed when the fluid is far from its linear viscoelastic behavior.

Large slippage and depletion layer at the polyelectrolyte/solid interface.

The slippage of polymer solutions on solid surfaces is often attributed to a depletion layer whose origin, thickness, and interaction with the flow are poorly understood. Using a Dynamic Surface Force Apparatus we report a structural and nanorheological study of the interface between hydrolyzed poly-acrylamide solutions and platinum surfaces. Polyelectrolyte chains adsorb on the surfaces in a thin charged layer, acting as a nonattractive wall for the bulk solution. We investigate the flow of the visco-elastic solution on the adsorbed layer from the nanometer to 10 micrometers, bridging microscopic to macroscopic properties. At distances larger than 200 nanometers, the flow is well described by an apparent slip boundary condition. At smaller distance the apparent slip is found to decrease with the gap. In contrast to the apparent slip model, we show that a 2-fluids model taking into account the finite thickness of depletion layers at the non-attractive wall describes accurately the dynamic forces over 4 spatial decades of confinement. Depletion layers are found to be an equilibrium property of the interface, independent on the flow and on the confinement. Their thickness is phenomenologically described by ξ + 2lD with ξ the correlation length of the semi-dilute solutions and lD the Debye length. We interpret this result in terms of screened repulsion between the charged adsorbed layer and the bulk polyions.

Nanorheology with a Conventional Rheometer: Probing the Interfacial Properties in Compatibilized Multinanolayer Polymer Films.

Measuring the viscoelastic behavior of polymers in the vicinity of a surface or under confinement is an experimental challenge. Simple rheological tests of nanolayered films of polyethylene/polyamide 6 compatibilized in situ during the coextrusion process enabled the probing of these interfacial properties. Taking advantage of the different melting points and of the multiplication of the number of interfaces, a drastic increase of dynamic moduli was reported when increasing the interphase volume fraction in the films. A solid-like behavior for the interphase was identified. The complex viscosity of nanolayered films as a function of angular frequency was quantitatively captured for all samples using a weighted mixing law of bulk and interphase viscosities, without additional adjusting parameters, highlighting the interfacial synergy developed in nanolayered polymer films.

Temperature-Controlled Slip of Polymer Melts on Ideal Substrates.

The temperature dependence of the hydrodynamic boundary condition between a polydimethylsiloxane melt and two different nonattractive surfaces made of either an octadecyltrichlorosilane self-assembled monolayer or a grafted layer of short polydimethylsiloxane chains has been characterized. We observe a slip length proportional to the fluid viscosity. The temperature dependence is deeply influenced by the surfaces. The viscous stress exerted by the polymer liquid on the surface is observed to follow exactly the same temperature dependences as the friction stress of a cross-linked elastomer sliding on the same surfaces. Far above the glass transition temperature, these observations are rationalized in the framework of a molecular model based on activation energies: increase or decrease of the slip length with increasing temperatures can be observed depending on how the activation energy of the bulk viscosity compares to that of the interfacial Navier's friction coefficient.

Sensing adsorption kinetics through slip velocity measurements of polymer melts.

The evolution over time of the nonlinear slip behavior of a polydimethylsiloxane (PDMS) polymer melt on a weakly adsorbing surface made of short non-entangled PDMS chains densely end-grafted to the surface of a fused silica prism has been measured. The critical shear rate at which the melt enters the nonlinear slip regime has been shown to increase with time. The adsorption kinetics of the melt on the same surface has been determined independently using ellipsometry. We show that the evolution of slip can be explained by the slow adsorption of melt chains using the Brochard-de Gennes's model.

Influence of Evaporation on Soap Film Rupture.

Although soap films are prone to evaporate due to their large surface to volume ratio, the effect of evaporation on macroscopic film features has often been disregarded in the literature. In this work, we experimentally investigate the influence of environmental humidity on soap film stability. An original experiment allows to measure both the maximum length of a film pulled at constant velocity and its thinning dynamics in a controlled atmosphere for various values of the relative humidity [Formula: see text]. At first order, the environmental humidity seems to have almost no impact on most of the film thinning dynamics. However, we find that the film length at rupture increases continuously with [Formula: see text]. To rationalize our observations, we propose that film bursting occurs when the thinning due to evaporation becomes comparable to the thinning due to liquid drainage. This rupture criterion turns out to be in reasonable agreement with an estimation of the evaporation rate in our experiment.