Department of Mechanical Engineering, Indian Institute of Science, Bangalore 560 012, India.
Langmuir. 2009 Oct 20;25(20):12120-6. doi: 10.1021/la902679c.
A rough hydrophobic surface when immersed in water can result in a "Cassie" state of wetting in which the water is in contact with both the solid surface and the entrapped air. The sustainability of the entrapped air on such surfaces is important for underwater applications such as reduction of flow resistance in microchannels and drag reduction of submerged bodies such as hydrofoils. We utilize an optical technique based on total internal reflection of light at the water-air interface to quantify the spatial distribution of trapped air on such a surface and its variation with immersion time. With this technique, we evaluate the sustainability of the Cassie state on hydrophobic surfaces with four different kinds of textures. The textures studied are regular arrays of pillars, ridges, and holes that were created in silicon by a wet etching technique, and also a texture of random craters that was obtained through electrodischarge machining of aluminum. These surfaces were rendered hydrophobic with a self-assembled layer of fluorooctyl trichlorosilane. Depending on the texture, the size and shape of the trapped air pockets were found to vary. However, irrespective of the texture, both the size and the number of air pockets were found to decrease with time gradually and eventually disappear, suggesting that the sustainability of the "Cassie" state is finite for all the microstructures studied. This is possibly due to diffusion of air from the trapped air pockets into the water. The time scale for disappearance of air pockets was found to depend on the kind of microstructure and the hydrostatic pressure at the water-air interface. For the surface with a regular array of pillars, the air pockets were found to be in the form of a thin layer perched on top of the pillars with a large lateral extent compared to the spacing between pillars. For other surfaces studied, the air pockets are smaller and are of the same order as the characteristic length scale of the texture. Measurements for the surface with holes indicate that the time for air-pocket disappearance reduces as the hydrostatic pressure is increased.
当粗糙的疏水性表面浸入水中时，会导致“Cassie”润湿状态，其中水与固体表面和被困住的空气同时接触。在水下应用中，如微通道中减少流动阻力和水翼等水下物体的阻力减少，被困住的空气的可持续性是很重要的。我们利用基于水-气界面全内反射的光学技术来量化这种表面上被困住的空气的空间分布及其随浸入时间的变化。利用这种技术，我们评估了具有四种不同纹理的疏水性表面上 Cassie 状态的可持续性。研究的纹理是通过湿法刻蚀技术在硅中形成的规则排列的柱子、脊和孔，以及通过铝的电火花加工获得的随机火山口纹理。这些表面通过自组装的氟辛基三氯硅烷层被赋予疏水性。根据纹理的不同，被困住的空气袋的大小和形状会有所不同。然而，无论纹理如何，空气袋的大小和数量都随着时间的推移逐渐减少，最终消失，这表明所有研究的微结构的“Cassie”状态的可持续性是有限的。这可能是由于空气从被困住的空气袋扩散到水中。空气袋消失的时间尺度取决于微结构的种类和水-气界面的静水压力。对于具有规则排列的柱子的表面，空气袋以薄层的形式栖息在柱子的顶部，与柱子之间的间隔相比具有较大的横向延伸。对于研究的其他表面，空气袋较小，与纹理的特征长度尺度相同。对于具有孔的表面的测量表明，随着静压的增加，空气袋消失的时间减少。
