MNHT 2008-52096 SESSILE DROP EVAPORATION ON SURFACES OF VARIOUS WETTABILITY

ABSTRACT This work experimentally investigates the evaporation rates of water drops on surfaces of various wettability. By measuring the temporal evolutions of the drop radius and contact angle, we find the qualitative difference between the evaporation behavior on hydrophilic surfaces where the contact radius remains constant initially and that on the superhydrophobic surfaces where the contact angle remains constant. Also, the evaporation rate is observed to depend on the surface material although the currently available models assume that the rate is solely determined by the drop geometry. Although the theory to explain this dependence on the surface remains to be pursued by the future work, we give the empirical relations that can be used to predict the drop volume evolution for each surface

Water-repellent soil and its relationship to granularity, surface roughness and hydrophobicity: a materials science view

Considerable soil water repellency has been observed at a wide range of locations worldwide. The soil exhibiting water repellency is found within the upper part of the soil profile. The reduced rate of water infiltration into these soils leads to severe runoff erosion, and reduction of plant growth. Soil water repellency is promoted by drying of soil, and can be induced by fire or intense heating of soil containing hydrophobic organic matter. Recent studies outside soil science have shown how enhancement of the natural water repellency of materials, both porous and granular, by surface texture (i.e. surface roughness, pattern and morphology) into super-hydrophobicity is possible. The similarities between these super-hydrophobic materials and observed properties of water-repellent soil are discussed from a non-soil scientist, materials-based perspective. A simple model is developed for a hydrophobic granular surface and it is shown that this can provide a mechanism for enhancement of soil water repellency through the relative size and spacing of grains and pores. The model provides a possible explanation for why soil water repellency should be more prevalent under dry conditions than wet. Consequences for water runoff, raindrop splash and soil erosion are discussed

Thermosensitive PNIPAM grafted alginate/chitosan PEC

Smart biomaterial functionality such as controlled adhesion properties is crucial to limit strip-off injuries. Among functional polymers, poly-N(isopropylacrylamide) (PNIPAM) allows surface properties to be changed depending on the temperature, with a transition of its properties that occurs around 32 °C, called the lower critical solution temperature (LCST). This transition is expected to modify surface interactions. Alginate and chitosan are biocompatible polymers commonly combined as polyelectrolyte complex (PEC) and are suitable for wound dressing applications. As a complex system, however, it is not so trivial to achieve an efficient functionalization. Herein, we elaborated a procedure to functionalize the surface of alginate/chitosan PECs without altering their intrinsic properties. FTIR revealed that acidic treatment led to a partial decomplexation of the PECs. Therefore, while the N-Hydroxysuccinimide/N-(3-Dimethylaminopropyl)-N′-ethylcarbodiimide (NHS/EDC) coupling usually requires an intermediate pH, we showed that a preliminary acidification seemed to increase the surface grafting efficiency. Water contact angle increased from 51° to 72°, showing that PNIPAM enhanced the surface hydrophobicity. The LCST transition modified the interaction forces between PNIPAM and model surfaces: it revealed an unexpected thermosensitive behaviour as hydrophobic transition favoured interactions with hydrophilic surfaces. It was presumably due to PNIPAM/PEC substrate interactions. Finally, the surface modification did not affect the release properties of the PEC biomaterial

Simulation of bubbles

International audienceWe present a novel framework based on a continuous fluid simulator for general simulation of realistic bubbles, with which we can handle as many significant dynamic bubble effects as possible. To capture a very thin liquid film of bubbles, we have developed a regional level set method allowing multi-manifold interface tracking. Based on the definitions of regional distance and its five operators, the implementation of the regional level set method is very easy. An implicit surface of liquid film with arbitrary thickness can be reconstructed from the regional level set function. To overcome the numerical instability problem, we exploit a new semi-implicit surface tension model which is unconditionally stable and makes the simulation of surface tension dominated phenomena much more efficient. An approximated film thickness evolution model is proposed to control the bubble's lifecycle. All these new techniques combine into a general framework that can produce various realistic dynamic effects of bubbles

General Predictive Framework for Droplet Detachment Force

Liquid droplets hanging from solid surfaces are commonplace, but their physics is complex. Examples include dew or raindrops hanging onto wires or droplets accumulating onto a cover placed over warm food or windshields. In these scenarios, determining the force of detachment is crucial to rationally design technologies. Despite much research, a quantitative theoretical framework for detachment force remains elusive. In response, we interrogated the elemental droplet surface system via comprehensive laboratory and computational experiments. The results reveal that the Young Laplace equation can be utilized to accurately predict the droplet detachment force. When challenged against experiments with liquids of varying properties and droplet sizes, detaching from smooth and microtextured surfaces of wetting and non wetting chemical makeups, the predictions were in an excellent quantitative agreement. This study advances the current understanding of droplet physics and will contribute to the rational development of technologies