University of Twente Student Theses


Dissolution modes of droplets on patterned surfaces

Jimidar, I.S.M. (2016) Dissolution modes of droplets on patterned surfaces.

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Abstract:Evaporating droplets have many applications within the industry and droplets evaporating in air have been extensively studied. However droplets surrounded by another liquid have been studied less. Due to the emergence of the micro- and nanotechnology many research groups have shown interest in the contact line dynamics of surface nanodroplets and nanobubbles. Understanding of the contact line dynamics will be beneficial and relevant to diverse industries. Surface droplets dissolve in four different modes: the constant radius (CR) mode, the constant contact angle (CA) mode, the stick-slide mode and the stick-jump mode. In this thesis we study the dissolution of these sessile droplets both numerically and experimentally. We employ the lattice Boltzmann model together with an evaporation model that has been developed by Hessling et al. \cite{hessling} to study the contact line dynamics of dissolving sessile droplets on chemically patterned surfaces. At first we study a droplet placed in the system center in still fluid, i.e. we study the classical Epstein-Plesset \cite{epstein} problem. From this benchmark we learn that the measured diffusion constant is not equal to the actual diffusivity in the system. We find that as the distance of the droplet to the boundary of the system increases, the measured diffusivity gets smaller. This is due to the infinite system size assumption in the theory. By patterning surfaces we are able to study the dissolution modes of surface droplets. We achieve the CA mode by simulating droplets on flat surfaces with different wettability. We simulate droplets dissolve in the CR mode, by depositing the droplet on a hydrophylic disk, and the droplet gets pinned at the rim of the hydrophylic disk. The rest of the surface is hydrophobic. When the droplet depins, the droplet further dissolves in the CA mode. Thus this resembles what we call the stick-slide mode. We find that the droplets dissolve faster in the CR mode than the CA mode. We then continue by patterning the surface with concentric rings, which consists of hydrophylic and hydrophobic rings. We deposit the droplet on the outer most hydrophylic ring, and find that the droplet depins and thereafter ``jumps" to the next available hydrophylic disk on the surface. This implies that the droplet dissolve in the stick-jump mode, if there are more hydrophylic disks on the surface. Experimentally we prepare the samples with an chemical etching process which leads to pyramidal hillock formation on the samples. We measure the contact angle hysteresis. Due to random distribution of the pyramidal hillocks on the surface, the measurements result in a large spread of the data, i.e. we do not find any consistent result between etching time and contact angle hysteresis. On average the hysteresis does increase after etching the samples. Furthermore from dissolution experiments we find that the droplets solely dissolve in the stick-slide mode and the stick-jump mode. Also from this data we do not find any consistency between pinning behaviour and etching time of the samples.
Item Type:Essay (Master)
Faculty:TNW: Science and Technology
Subject:33 physics
Programme:Applied Physics MSc (60436)
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