Pinning–Depinning Mechanism of the Contact Line during Evaporation of Nanodroplets on Heated Heterogeneous Surfaces: A Molecular Dynamics Simulation

Abstract

Droplet evaporation on heterogeneous or patterned surfaces has numerous potential applications, for example, inkjet printing. The effect of surface heterogeneities on the evaporation of a nanometer-sized cylindrical droplet on a solid surface is studied using molecular dynamics simulations of Lennard-Jones particles. Different heterogeneities of the surface were achieved through alternating stripes of equal width but two chemical types, which lead to different contact angles. The evaporation induced by the heated substrate instead of the isothermal evaporation is investigated. It is found that the whole evaporation process is generally dominated by the nonuniform evaporation effect. However, at the initial moment, the volume expansion and local evaporation effects play important roles. From the nanoscale point of view, the slow movement of the contact line during the pinning process is observed, which is different from the macroscopic stationary pinning. Particularly, we found that the speed of the contact line may be not only affected by the intrinsic energy barrier between the two adjacent stripes (ũ) but also relevant to the evaporation rate. Generally speaking, the larger the intrinsic energy barrier, the slower the movement of the contact line. At the specified temperature, when ũ is less than a critical energy barrier (ũ*), the speed of the contact line would increase with the evaporate rate. When ũ > ũ*, the speed of the contact line is determined only by ũ and no longer affected by the evaporation rate at different stages (the first stick and the second stick).

Controlling the pinning time of a receding contact line under forced wetting conditions

“…There, the contact line moves along the hydrophobic stripe during the evaporation (equivalent to our region I). After some time, the contact line reaches the hydrophobic-hydrophilic boundary and it remains pinned while the contact angle changes (CCR equivalent to our region II) although the observation at the nanoscale of this region suggests that, at the molecular level, pinning is actually a strong decrease in the contact line velocity and not an absence of displacement as already described in literature [37]. When the contact angle reaches a critical value, the contact line continues receding across the hydrophilic patch and the contact angle eventually reaches a constant value (CCA equivalent to our region III).…”

“…Once the BL contact line reaches the heterogeneity region II starts where the contact line drastically reduces its velocity (quasi-pinning) and the associated contact angle h BL ðtÞ decreases with time which is very similar to the ''Constant Contact Radius" (CCR) region observed in the modeling of drop evaporation over a patterned substrate [25,26,37] characterized by a constant value of the contact radius and a variation of the contact angle. The length of this time period is characterized by the pinning time s p defined as the time required for the receding contact angle on the bottom plate to evolve from its stationary value in the absence of the patch h st b to the equilibrium contact angle of the liquid on the patch h 0 p as represented in Fig.…”

“…In their pioneering work, Picknett and Bexon [36] identified two regimes in droplets evaporation on smooth substrates: the constant contact radius region (CCR) where the contact line is pinned to the solid substrate (similar to our region II) and the constant contact angle region (CCA) where the contact angle remains constant while the contact line recedes (similar to our region III). More recent works have extended this study to the analysis of the pinning-depinning transition (CCR-CCA transition) that appears in spontaneous drop evaporation on substrates patterned with a series of hydrophobic and hydrophilic stripes [25,26,37]. There, the contact line moves along the hydrophobic stripe during the evaporation (equivalent to our region I).…”

“…Once h BL reaches the value of the equilibrium contact angle of the heterogeneity h 0 p , the BL contact line increases drastically its velocity which corresponds to the depinning transition. It is tempting to identify this region III as the ''Constant Contact Angle" (CCA) region observed in the modeling of drop evaporation over a pat-terned substrate [25,26,37] characterized by a constant value of the contact angle and a variation of the contact line position. The classical method to model this phenomenon corresponds to a quasi-static process where the contact line velocity does not affect the value of the contact angles.…”

Dynamic Defrosting on Superhydrophobic and Biphilic Surfaces

“…Contact lines are not restricted strictly to the HL stripes but instead present at the junction of SHP and HL regions. 72 The contact angle at this contact line can thus be higher than that of HL stripes. In fact, it can be as high as that of the SHP region.…”