Xu Xinpeng, Qian Tiezheng
Nano Science and Technology (NSNT) Program, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong.
Phys Rev E Stat Nonlin Soft Matter Phys. 2012 Jun;85(6 Pt 1):061603. doi: 10.1103/PhysRevE.85.061603. Epub 2012 Jun 26.
Using a continuum model capable of describing the one-component liquid-gas hydrodynamics down to the contact line scale, we carry out numerical simulation and physical analysis for the droplet motion driven by thermal singularity. For liquid droplets in one-component fluids on heated or cooled substrates, the liquid-gas interface is nearly isothermal. Consequently, a thermal singularity occurs at the contact line and the Marangoni effect due to temperature gradient is suppressed. Through evaporation or condensation in the vicinity of the contact line, the thermal singularity makes the contact angle increase with the increasing substrate temperature. This effect on the contact angle can be used to move the droplets on substrates with thermal gradients. Our numerical results for this kind of droplet motion are explained by a simple fluid dynamical model at the droplet length scale. Since the mechanism for droplet motion is based on the change of contact angle, a separation of length scales is exhibited through a comparison between the droplet motion induced by a wettability gradient and that by a thermal gradient. It is shown that the flow field at the droplet length scale is independent of the statics or dynamics at the contact line scale.
使用一种能够描述单组分液-气流体动力学直至接触线尺度的连续介质模型，我们对由热奇点驱动的液滴运动进行了数值模拟和物理分析。对于加热或冷却基板上的单组分流体中的液滴，液-气界面几乎是等温的。因此，在接触线处会出现热奇点，并且由于温度梯度引起的马兰戈尼效应受到抑制。通过接触线附近的蒸发或冷凝，热奇点使接触角随基板温度的升高而增大。这种对接触角的影响可用于在具有热梯度的基板上移动液滴。我们针对这种液滴运动的数值结果由液滴长度尺度下的一个简单流体动力学模型进行了解释。由于液滴运动的机制基于接触角的变化，通过比较由润湿性梯度引起的液滴运动和由热梯度引起的液滴运动，展现出了长度尺度的分离。结果表明，液滴长度尺度处的流场与接触线尺度处的静态或动态无关。
