Experimental Investigation of Evaporation from Low-Contact-Angle Sessile Droplets

Abstract

Evaporating sessile drops remain pinned at the contact line during much of the evaporation process, and leave a ring of residue on the surface upon dryout. The intensive mass loss near the contact line causes solute particles to flow to the edge of the droplet and deposit at the contact line. The high vapor diffusion gradient and the low thermal resistance of the film near the contact line are responsible for very efficient mass transfer in this region. Although heat and mass transfer at the contact line have been extensively studied, well-characterized experiments remain scarce. The local mass transport in a 100-400 microm region near the contact line of a water droplet of radius 1810 microm on a glass substrate is experimentally quantified in the present work. Microparticle image velocimetry measurements of the three-dimensional flow field near the contact line are conducted to map the velocity field. Combined with high-resolution transient liquid profile shapes, the measured velocity field yields transient local evaporative mass fluxes near the contact line. The spatial and temporal distribution of the local evaporative flux is also documented. The temperature distribution in the droplet near the contact line is deduced from the local evaporative fluxes and interface mass transport theory.

Characterization of ultrahydrophobic hierarchical surfaces fabricated using a single-step fabrication methodology

“…The interesting observation here is that sliding of the contact line begins when the contact angle is equal to the receding contact angle. This observation is similar to that in the case of hydrophilic surfaces [23] and smooth hydrophobic surfaces. For all the three surfaces under consideration this angle is in close agreement with receding angles obtained from the roll-off experiments (Table 4).…”

Assessment of Nanostructured Capillary Wicks for Passive Two-Phase Heat Transport

“…The capillary pressure ΔP cap generated by a wick depends on the mean curvature of the liquid meniscus H formed in its pore and the surface tension γ LV of the liquid, as given by the Young-Laplace equation [29]: 12 11 2…”

“…Liquid molecules in the thin-film region experience intermolecular attractive forces with the solid, leading to a favorable pressure gradient for evaporation. Evaporation occurring from the thin-film region of the meniscus has been shown to contribute as much as 60-70% of the total heat transfer occurring from the meniscus [7,[10][11][12].…”

Velocity field measurements in an evaporating sessile droplet by means of micro-PIV technique

“…3. One can see that at the beginning of evaporation flow structure does not present typical radial flow [5]. Instead, there are two symmetrical vortices in the droplet, which presumably are connected with some local depinning of the contact line.…”

“…The authors of the work [4] used the ray tracing method in order to correct optical disturbances during PIV measurements from the side view of the droplet. In the work [5] micro-PIV technique was used for velocity field measurements in the small region near the contact line of evaporating water droplet on a glass substrate. In the present work in order to visualize flow structure in evaporating droplets on the substrates with different contact angles micro-PIV technique is applied.…”