Dynamics and mechanism of liquid film collapse in a foam

Collapse of liquid films in a foam is characteristic compared to that of individual films. The mechanism and physical conditions of collective bubble collapse which directly contribute to foam stability have been clarified.

Lacritin proteoforms prevent tear film collapse and maintain epithelial homeostasis

Lipids in complex, protein-enriched films at air/liquid interfaces reduce surface tension. In the absence of this benefit, the light refracting and immunoprotective tear film on eyes would collapse. Premature collapse, coupled with chronic inflammation compromising visual acuity, is a hallmark of dry eye disease affecting 7 – 10% of individuals worldwide. Although collapse seems independent of mutation (unlike newborn lung alveoli), selective proteome and possible lipidome changes have been noted. These include elevated tissue transglutaminase and consequent inactivation through C-terminal cross-linking of the tear mitogen lacritin, leading to significant loss of lacritin monomer. Lacritin monomer restores homeostasis via autophagy and mitochondrial fusion and promotes basal tearing. Here, we discover that lacritin monomer C-terminal processing, inclusive of cysteine, serine and metalloproteinase activity, generates cationic amphipathic α-helical proteoforms. Such proteoforms (using synthetic peptide surrogates) act like alveolar surfactant proteins to rapidly bind and stabilize the tear lipid layer. Immunodepletion of C-, but not N-terminal proteoforms nor intact lacritin, from normal human tears promotes loss of stability akin to human dry eye tears. Stability of these and dry eye tears is rescuable with C- but not N-terminal proteoforms. Repeated topical application in rabbits reveals a proteoform turnover time of 7 – 33 hours with gradual loss from human tear lipid that retains bioactivity without further processing. Thus, the processed C-terminus of lacritin that is deficient or absent in dry eye tears appears to play a key role in preventing tear film collapse and as a natural slow release mechanism that restores epithelial homeostasis.

A new wrinkle on liquid sheets: Turning the mechanism of viscous bubble collapse upside down

Viscous bubbles are prevalent in both natural and industrial settings. Their rupture and collapse may be accompanied by features typically associated with elastic sheets, including the development of radial wrinkles. Previous investigators concluded that the film weight is responsible for both the film collapse and wrinkling instability. Conversely, we show here experimentally that gravity plays a negligible role: The same collapse and wrinkling arise independently of the bubble’s orientation. We found that surface tension drives the collapse and initiates a dynamic buckling instability. Because the film weight is irrelevant, our results suggest that wrinkling may likewise accompany the breakup of relatively small-scale, curved viscous and viscoelastic films, including those in the respiratory tract responsible for aerosol production from exhalation events.

The thermo-wetting instability driving Leidenfrost film collapse

Above a critical temperature known as the Leidenfrost point (LFP), a heated surface can suspend a liquid droplet above a film of its own vapor. The insulating vapor film can be highly detrimental in metallurgical quenching and thermal control of electronic devices, but may also be harnessed to reduce drag and generate power. Manipulation of the LFP has occurred mostly through experiment, giving rise to a variety of semiempirical models that account for the Rayleigh–Taylor instability, nucleation rates, and superheat limits. However, formulating a truly comprehensive model has been difficult given that the LFP varies dramatically for different fluids and is affected by system pressure, surface roughness, and liquid wettability. Here, we investigate the vapor film instability for small length scales that ultimately sets the collapse condition at the Leidenfrost point. From a linear stability analysis, it is shown that the main film-stabilizing mechanisms are the liquid–vapor surface tension-driven transport of vapor mass and the evaporation at the liquid–vapor interface. Meanwhile, van der Waals interaction between the bulk liquid and the solid substrate across the vapor phase drives film collapse. This physical insight into vapor film dynamics allows us to derive an ab initio, mathematical expression for the Leidenfrost point of a fluid. The expression captures the experimental data on the LFP for different fluids under various surface wettabilities and ambient pressures. For fluids that wet the surface (small intrinsic contact angle), the expression can be simplified to a single, dimensionless number that encapsulates the wetting instability governing the LFP.

Discerning Wine Astringency Sub-Qualities by Tribological Approaches in a Model System—What Is the Role of Saliva?

Astringency is an important indicator of wine sensory quality. As a complex sensation that the loss of in-mouth lubrication is recognised as an important mechanism, astringency can be perceived as a set of different sub-qualities. In addition to sensory evaluations, chemical approaches measuring interactivity of astringents-proteins have provide ways of astringency quantification, but they do not capture how specific wine components elicit those sub-qualities. In this study, we developed several tribological approaches, which considered saliva interactions, in an effort to explore if any salivary lubrication change is linked to different sensorily perceived wine astringency sub-qualities. In model wine systems with defined matrix compositions, we found that model wines with equivalent responses in chemical assay for astringency possessed various astringency sub-qualities. In particular, the sub-quality drying corresponded to samples exhibiting higher boundary friction. High-acidity matrix induced pucker, which is found to be independent of drying. It was also linked to a faster salivary film collapse. The rough seemed to be a secondary astringency sub-quality combining drying and pucker, because these 2 sensations were found either elicit rough independently. Polysaccharide reduced the drying in a low-tannin-high-acidity matrix. This is the first study demonstrating that astringency sub-qualities are likely to have different perceptive mechanisms by using tribological measures. The outcomes would provide insights into employing those tools to help manage wine matrix to attain preferable mouthfeels and astringency characteristics.

Thermocapillary instability as a mechanism for film boiling collapse

We construct a model to investigate the interfacial stability of film boiling, and discover that instability of very thin vapour films and subsequent large interface superheating is only possible if thermocapillary instabilities are present. The model concerns horizontal saturated film boiling, and includes novel features such as non-equilibrium evaporation based on kinetic theory, thermocapillary and vapour thrust stresses and van der Waals interactions. From linear stability analysis applied to this model, we are led to suggest that vapour film collapse depends on a balance between thermocapillary instabilities and vapour thrust stabilization. This yields a purely theoretical prediction of the Leidenfrost temperature. Given that the evaporation coefficient is in the range 0.7–1.0, this model is consistent with the average Leidenfrost temperature of every fluid for which data could be found. With an evaporation coefficient of 0.85, the model can predict the Leidenfrost point within 10 % error for every fluid, including cryogens and liquid metals where existing models and correlations fail.