| Summary: | 碩士 === 國立中央大學 === 化學工程與材料工程學系 === 105 === Liquid drops spontaneously spreading on a solid surface is referred to as the total wetting phenomenon. A typical example is the continuous expansion of a silicon drop on various surfaces. The time evolution of the drop radius can be described by the power law r(t) ∼ tn with the exponent 1/10, known as Tanners law. In this work, the spreading dynamics of a nanodrop on a total wetting surface under saturated humidity and no-slip condition has been explored by many-body dissipative particle dynamics simulation. The influences of surface roughness and wettability in terms of spreading coefficient on the spontaneous spreading behavior are studied.
It is found that the exponent n is 0.2 on smooth surface, and the exponent grows with increasing the spreading coefficient. The reason for the difference with Tanner’s law is that the formation of precursor film with the drop spreading process. As we remove the effect of the precursor film, the drop radius and contact angle behavior agree with Tanner’s law and the spreading law is independent of the spreading coefficient. In addition, The drop spreading velocity on rough surface is lower than that on smooth surface and the spreading process also follow Tanner’s law. It shows the surface roughness will prevent the growing of the precursor film and the resistance rises with increasing the cavity size. Furthermore, the drop spreading velocity is significantly lower and the drop becomes very thin for a long time because the most of the liquid beads stuck in the cavity.
The forced spreading that is similar to the thin film flow on the surface under external force are investigated as well. As a result, the drop spreading motion which is vertical to external force also consistent with Tanner’s law and the precursor film is produced in the spreading process with this direction. On the forced direction, the rear part of the liquid drop will slide on smooth surface due to without surface roughness that can make pinning force to impede the movement of the contact line. In contrast, the rear part of the liquid drop will pinning on rough surface. Moreover, the precursor film can’t be observed on forced direction, since the forced flow velocity is faster than the velocity of expansion with precursor. In addition, the flow velocity with the forced direction will be affected by the precursor and the surface roughness will make the flow velocity significant
slower.
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