Improving a pavement-watering method on the basis of pavement surface temperature measurements

Pavement-watering has been studied since the 1990's and is currently considered a promising tool for urban heat island reduction and climate change adaptation. However, possible future water resource availability problems require that water consumption be optimized. Although pavement heat flux can be studied to improve pavement-watering methods (frequency and water consumption), these measurements are costly and require invasive construction work to install appropriate sensors in a dense urban environment. Therefore, we analyzed infrared camera measurements of pavement surface temperatures in search of alternative information relevant to this goal. Firstly, surface temperature reductions of up to 4{\textdegree}C during shading and 13{\textdegree}C during insolation were found. Secondly, the infrared camera successfully detected temperature spikes indicative of surface drying and can therefore be used to optimize the watering frequency. Measurements made every 5 min or less are recommended to minimize relevant data loss. Finally, if the water retaining capacity of the studied pavement is known, optimization of total water consumption is possible on the sole basis of surface temperature measurements.Comment: Published in Urban Climat

The effect of pavement-watering on subsurface pavement temperatures

International audiencePavement-watering is currently viewed as a potential climate change adaptation and urban heat island mitigation technique. The effects of pavement-watering on pavement temperature measured 5 cm deep are presented and discussed. Subsurface temperature measurements could not be used to improve or optimize pavement-watering methods as was seen in previous work on surface temperatures or subsurface pavement heat flux measurements

A simple model for the dynamics of contact lines freezing on a cold substrate

International audienceStarting from de Gennes model of contact line dynamics, we develop a simple, analytical, description of a four phase contact line, at which a liquid advances on a cold substrate (of infinite therml conductivity) and at the same time freezes on the solid. The key idea consists in a subtle balance between thermical and hydrodynamical divergences at contact line, heat produced by dissipation in the liquid phase limitating the solid growth. We calculate the apparent liquid contact angle and its equivalent for the solid phase as a function of the contact line velocity U and the other parameters involved, in the limit of small angles. We show that the arrest condition (U=0) implies a divergence of these angles, that are presumably reaching 90°, while the liquid film thickness vanishes, which is reminiscent of previous works of Sonin et al in the 90's. Remarkably, at low contact line velocity, we show that a stick-slip behaviour is to be expected, which has been recently observed experimentally. The temperature dependance of the critical velocity differs from what is found in recent experiments, but this model is to our opinion a necessary first step to understand these complex behaviours

Dynamics of spreading of liquid on a hydrogel substrate

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Dynamics of the contact line in wetting and diffusing processes of water droplets on hydrogel (PAMPS–PAAM) substrates

International audienceWe studied the dynamics of the wetting and diffusing processes of water droplets on hydrogel (Poly(2-acrylamido-2-methyl-propane-sulfonic acid-co-acrylamide) (PAMPS-PAAM)) substrates. The profiles of the droplet and substrate were measured simultaneously using a grid projection method. We observed that as the water droplet diffuses into the gel, the contact line of the droplet exhibits successively two different behaviors: pinned and receding, and the transition between these two behaviors is closely related to the local deformation of the gel substrate. The contact line is pinned at an early stage. As the water diffusion proceeds, the contact angle of the droplet decreases while the angle of the local slope of the gel surface near the contact line increases. At the moment where these two angles almost correspond to each other, the contact line starts to recede. Our results indicate that due to the water diffusion, a locally swollen region is formed in the vicinity of the droplet-gel interface, and whether the contact line is pinned or recedes is determined by the surface property of this swollen region

Unveiling of the mechanisms of acoustic streaming induced by sharp edges

Acoustic waves can generate steady streaming within a fluid owing to the generation of viscous boundary layers near walls, of typical thickness $\delta$. In microchannels, the acoustic wavelength $\lambda$ is adjusted to twice the channel width $w$ to ensure a resonance condition, which implies the use of MHz transducers. Recently though, intense acoustic streaming was generated by acoustic waves of a few kHz (hence with $\lambda \gg w$), owing to the presence of sharp-tipped structures of curvature radius at the tip $r_c$ smaller than $\delta$. The present study quantitatively investigates this sharp-edge acoustic streaming via the direct resolution of the full Navier-Stokes equation, using Finite Element Method. The influence of $\delta$, $r_c$ and viscosity $\nu$ on the acoustic streaming performance are quantified. Our results suggest choices of operating conditions and geometrical parameters, via dimensionless quantities $r_c/\delta$ and $\delta/w$ and provide guidelines on how to obtain strong, optimal sharp-edge acoustic streaming.Comment: 33 pages, 11 figure

Edge effects on water droplet condensation

International audienceIn this study is investigated the effect of geometrical or thermal discontinuities on the growth of water droplets condensing on a cooled substrate. Edges, corners, cooled/non cooled boundaries can have a strong effect on the vapor concentration profile and mass diffusion around the drops. In comparison to growth in a pattern where droplets have to compete to catch vapor, which results in a linear water concentration profile directed perpendicularly to the substrate, droplets near discontinuities can get more vapor (outer edges, corners), resulting in faster growth or less vapor (inner edges), giving lower growth. When the cooling heat flux limits growth instead of mass diffusion (substrate with low thermal conductivity, strong heat exchange with air), edge effects can be canceled. In certain cases, growth enhancement can reach nearly 500% on edges or corners

Dynamics of Liquid Contact Line on Visco-Elastic Gels

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Drop spreading and gelation of thermoresponsive polymers

Spreading and solidification of liquid droplets are elementary processes of relevance for additive manufacturing. Here we investigate the effect of heat transfer on spreading of a thermoresponsive solution (Pluronic F127) that undergoes a sol-gel transition above a critical temperature $T_m$. By controlling the concentration of Pluronic F127 we systematically vary $T_m$, while also imposing a broad range of temperatures of the solid and the liquid. We subsequently monitor the spreading dynamics over several orders of magnitude in time and determine when solidification stops the spreading. It is found that the main parameter is the difference between the substrate temperature and $T_m$, pointing to a local mechanism for arrest near the contact line. Unexpectedly, the spreading is also found to stop below the gelation temparature, which we attribute to a local enhancement in polymer concentration due to evaporation near the contact line.Comment: 9 pages, 10 figure