How water droplets evaporate on a superhydrophobic substrate

Abstract

Evaporation of water droplets on a superhydrophobic substrate, on which the contact line is pinned, is investigated. While previous studies mainly focused on droplets with contact angles smaller than 90 • , here we analyze almost the full range of possible contact angles (10 • -150 • ). The greater contact angles and pinned contact lines can be achieved by the use of superhydrophobic Carbon Nanofiber substrates. The time-evolutions of the contact angle and the droplet mass are examined. The experimental data is in good quantitative agreement with the model presented by Popov (Physical Review E 71, 2005), demonstrating that the evaporation process is quasi-static, diffusion-driven, and that thermal effects play no role. Furthermore, we show that the experimental data for the evolution of both the contact angle and the droplet mass can be collapsed onto one respective universal curve for all droplet sizes and initial contact angles.

Evaporation of water: evaporation rate and collective effects

“…1 we plot J/(Dc s R d ) as a function of θ. Careful experiments with small partially wetting drops with pinned contact lines have confirmed the theoretical result (2.5) quantitatively over a wide range of contact angles (Gelderblom et al 2011). …”

“…While in this case it is often observed that drops are either pinned completely (i.e. the radius is constant, (Gelderblom et al 2011)) or perform a stick-slip motion (Cazabat & Guéna 2010), for our systems we observe the drop radius to shrink continuously during evaporation without pinning. We attribute this to our careful sample preparation, described in detail below.…”

“…As we will explain in more detail below, linear scaling with size results from the evaporation rate being governed by the diffusive transport through the vapor (Deegan et al 1997;Bonn et al 2009). The prefactor of this scaling law however depends on the shape of the drop, either through the contact angle (Gelderblom et al 2011;Sobac & Brutin 2011), or the fact that larger drops are flattened by gravity (Cazabat & Guéna 2010).…”

“…For an evaporating drop, one would guess naively that the surface area governs the evaporation rate; however for drops smaller than about a centimeter in a temperaturecontrolled environment, detailed experiments show that the rate is proportional to the drop radius R for both pinned (Deegan et al 2000b;Crafton & Black 2004;David et al 2007;Gelderblom et al 2011) and moving contact lines (Cachile et al 2002a,b;Poulard et al 2003;Shahidzadeh-Bonn et al 2006;Starov & Sefiane 2009). In fact, in the regime of slow evaporation considered here the evaporation is quasi-steady (Cazabat & Guéna 2010;Stauber et al 2015), hence evaporation rates are defined by the instantaneous drop shapes.…”

Pore- and micro-structural characterization of a novel structural binder based on iron carbonation

“…For Knudsen diffusion to be the limiting case, the mean free path needs to be greater than the average pore diameter in the media, and for normal diffusion to become dominant, the mean free path has to be smaller than the average pore diameter [40]. The diffusion coefficient of water vapor in air, Da, is 25 x 10 -6 m 2 /s [41,42]. The mean free path of water vapor () at standard temperature and pressure (25° C, 1 bar) is calculated as 0.12 µm using the following equation [43,44]:…”