Increased Evaporation Kinetics of Sessile Droplets by Using Nanoparticles

Abstract

The effect of nanoparticles on the evaporation of a sessile droplet into air is still controversial. Unlike insoluble surfactants which reduce the droplet evaporation rate, here we show that the presence of nanoparticles and the increase of their concentration lead to an increase in the overall rate of diffusive evaporation and, consequently, a decrease of the droplet lifetime. The nanoparticles accumulating at the droplet edge due to the well-known coffee-ring effect pin the three-phase contact line for an extended time and maintain a large air-water interface area, leading to the increased evaporation rate. We provide a full analytical prediction for the lifetime of a sessile droplet evaporating by the combined pinned-receding mode. A master equation and a master diagram for the droplet lifetime of the combined mode are obtained and experimentally validated, and explain the effect of nanoparticles on increasing the global evaporation rate and decreasing the droplet lifetime.

On the lifetimes of evaporating droplets

“…Note that we introduce the term 'stick-slide' (rather than use the term 'stick-slip' introduced by Shanahan (1995) to describe situations in which the 'slip' phases are of relatively short duration compared with the 'stick' phases) to emphasise that in the situations considered in the present work, the 'slip' and 'slide' phases may be of comparable duration. Various SS modes have been observed experimentally, but perhaps the most commonly reported SS mode is one (referred to as the 'combined pinned-receding mode' by Nguyen & Nguyen (2012b)) where there is an initial stick phase in which the droplet evaporates in a CR phase, followed by a first slide phase in which the droplet evaporates in a CA phase, followed in turn by a second slide phase in which both R and θ vary simultaneously. In practice, the second slide phase is often of relatively short duration compared with the stick and the first slide phases, in which case it can be neglected when determining the lifetime of the droplet.…”

“…In practice, the second slide phase is often of relatively short duration compared with the stick and the first slide phases, in which case it can be neglected when determining the lifetime of the droplet. Thus, in the present work we study a model for the SS mode, sketched in figure 1 (as discussed by, for example, Nguyen & Nguyen (2012b), Stauber et al (2013) and Dash & Garimella (2013)), in which initially the droplet evaporates in a CR phase with R = R 0 and with θ = θ(t) decreasing from θ = θ 0 (0 θ 0 π) to the receding contact angle θ = θ (referred to as the 'transition contact angle' by Nguyen & Nguyen (2012b)) (0 θ π), after which the droplet evaporates in a CA phase with θ = θ and with R = R(t) decreasing from R 0 to zero. The initial CR phase occurs only if θ 0 > θ ; otherwise the contact line is always de-pinned and the droplet simply evaporates in the CA mode.…”

“…Since the SS mode is a simple combination of the extreme modes, it would be natural to assume, as some previous authors (such as Nguyen & Nguyen (2012b)) have done, that the lifetime of a droplet evaporating in this mode is always constrained by the lifetimes of initially identical droplets evaporating in the extreme modes. However, while figures 3 and 4 show that when 0 < θ 0 π/2 (and, in particular, in the thin-film limit θ 0 → 0 + ) it is indeed correct that t SS lies between t CR and t CA , they also show that when π/2 < θ 0 < π this result is not, in general, correct.…”

On the lifetimes of evaporating droplets with related initial and receding contact angles

“…[6][7][8][9][10][11][12][13][14] is one in which an initial stick phase is followed by a first slide phase with constant contact angle and a second slide phase in which both the contact radius and the contact angle vary. In practice, the second slide phase can be relatively short compared to the other two phases, and so Nguyen and Nguyen, 15 Dash and Garimella, 14 and Stauber et al 16 considered a simple but effective model for an idealised SS mode in which the second slide phase is entirely neglected and initially the droplet evaporates in a CR phase in which R = R 0 and θ(t) and V (t) decrease until θ(t) reaches the receding contact angle θ * (0 ≤ θ * ≤ θ 0 ), at which the contact line depins and subsequently the droplet evaporates in a CA phase in which θ(t) = θ * and R(t) and V (t) decrease to zero at time t = t SS , where t SS (which depends on both θ 0 and θ * ) denotes the lifetime of the droplet. This mode of evaporation is sketched in Figure 1.…”

Influence of Particle Size on Evaporation Heat Transfer Characteristics of Nanofluid Droplet

“…특히 나노입자의 농도에 따라 시 편에 고착된 나노유체 액적이 증발할 때 다양한 동적 특성에 대하여 연구하였다 ( 5 ) . 나노입자의 다양한 종류에 따라서 나노유체의 열물성에 대하여 연구하였다 ( 6 ) .…”