Laboratory for Optics, Acoustics, and Mechanics, Department of Mechanical & Aerospace Engineering, Monash University, Clayton, VIC 3800, Australia.
J Colloid Interface Sci. 2011 Feb 15;354(2):832-42. doi: 10.1016/j.jcis.2010.11.027. Epub 2010 Dec 13.
The ability of a liquid droplet to move on an incline has important ramifications in discrete volume fluidic devices. By taking advantage of the spontaneous and copious formation of visible air bubbles within water droplets on a polytetrafluoroethylene (PTFE) surface, we uncovered a direct correlation between their presence and the ability of droplets to slide down an incline. We forward two possible mechanisms to account for this behavior. The first is attributed to the air bubbles creating regions where additional solid-liquid-vapor phase interfaces are present; wherein due to the buoyancy force acting upwards, the orientation of the contact angles of each bubble (which should also be in hysteresis but in the opposite direction of the hysteresis at the droplet rim contact lines) dictate that the net force of the bubbles in the droplet act down an incline. We show here that this mechanism cannot fully account for the bubble enhanced sliding behavior. The second mechanism is based on the occurrence of the droplet front advancing first, causing the droplet to elongate and thus allowing the receding contact line to partially sweep inwards over the bubbles. This causes a series of point-wise disruptions on the contact line that permits the droplet to slide down more readily. The relatively short time of ∼180s during which these micron sized bubbles decrease in size indicates a possibility of this mechanism contributing to a transient means to reduce the retention force of droplets that reside on hydrophobic surfaces.
液滴在斜面上移动的能力在离散体积流道装置中有重要的影响。通过利用聚四氟乙烯（PTFE）表面上液滴中可见空气泡的自发和丰富形成，我们发现了它们的存在与液滴沿斜面滑动能力之间存在直接相关性。我们提出了两种可能的机制来解释这种行为。第一种机制归因于气泡在液滴中形成了额外的固-液-气相间界面的区域；由于浮力向上作用，每个气泡的接触角（也应该处于滞后状态，但滞后方向与液滴边缘接触线的滞后方向相反）决定了气泡在液滴中的合力沿斜面向下。我们在这里表明，这种机制不能完全解释气泡增强滑动行为。第二种机制基于液滴前缘先前进的发生，导致液滴伸长，从而允许后退接触线部分向内扫过气泡。这在接触线上产生了一系列点式干扰，使得液滴更容易滑动。这些微米大小的气泡在大约 180 秒的时间内减小尺寸，这表明这种机制可能有助于减少驻留在疏水性表面上的液滴的保持力的短暂手段。
