Controlling Liquid Wetting of Textured Surfaces using Ultrasound

许多常见制造工艺的成功实施依赖于控制流体和液体在何时、何地以及如何润湿固体。这包括油漆、粘合、清洁、焊接和钎焊等过程。这种控制液体是否和何时湿润表面的能力在自清洁和抗污表面中也很重要。这种表面可以在各种应用中提供重要的技术和经济优势，例如，降低设备和建筑物（如海军舰艇）的维护和清洁成本，通过防止室外表面的雾霾和污垢积聚来提高太阳能电池的效率，以及降低物体在液体中移动时的摩擦。这个项目研究了高频振动，即超声波，如何根据表面纹理的尺寸尺度和振动频率之间的关系来改变表面上液体的润湿状态。纹理和应用振动的结合将使液体在何时何地润湿表面或从表面脱湿的控制成为可能。潜在的应用包括控制键合位置，促进键合成分的分离以进行回收，以及改善液体对粉末的润湿性。研究工作将与研究生和本科生的教育相结合。该项目还将通过新的振动和声学教育活动，加强与当地K-12学校的外展计划。纹理表面（表面粗糙的表面）在多种润湿状态下都是稳定的。当流体或液体渗透到纹理的凹槽（Wenzel状态）和流体停留在纹理的峰值顶部（Cassie状态）时，就会发生这两种极端情况。由于接触面积较大，温泽尔态具有更强的附着力。对于每种流体/基质组合，通常会有一种状态具有较低的能量，但是为了过渡到较低能量的湿态，必须克服一个能量屏障。先前的工作已经证明，可以通过以其特征频率振动液滴来实现过渡。然而，这在许多应用中是不切实际的，因为每个液滴都有不同的振动频率。这个项目将使用具有表面结构特征的频率，这样它们就可以在任意大小的液滴上工作。具体来说，该项目将确定不同频率下所需的能量，比较不同振动模式的有效性，并评估不对称振动对流体润湿的影响。这些目标将通过两种方法来实现：一种是实验测量在有图案的表面上移动液体湿润前沿的压力，另一种是对受振动影响的接触线进行数值模拟。

Successful implementation of many common manufacturing processes rely on controlling when, where, and how fluids and liquids wet a solid. This includes processes like painting, bonding, cleaning, soldering,and brazing. This ability to control if and when a liquid wets a surface is also of importance in self-cleaning and stain resistant surfaces. Such surfaces can provide important technological and economic advantages in a variety of applications by, for example, reducing maintenance and cleaning costs of equipment and buildings such as naval ships, improving efficiency of solar cells by preventing haze and dirt buildup on their surfaces outdoors, and lowering the friction of objects moving through a liquid. This project examines how high frequency vibrations, i.e., ultrasound, can be used to change the wetting state of a liquid on a surface based on the relationship between the size scale of the surface texture and the vibration frequency. The combination of texture and applied vibration will enable new control over where and when a liquid wets a surface or de-wets from it. Potential applications include controlling bonding locations, facilitating separation of bonded components for recycling, and improving the wetting of powders by liquids. The research efforts will be integrated with the education of both graduate and undergraduate students. The project will also be used to enhance outreach programs with local K-12 schools through new educational activities on vibration and acoustics. Textured surfaces (ones with surface roughness) can be stable in multiple wetting states. The two extreme conditions occur when a fluid or liquid penetrates into the recesses of the texture (Wenzel state) and when the fluid rests on top of the peaks of the texture (Cassie state). The Wenzel state has much stronger adhesion due to the larger contact area. For each fluid/substrate combination, one state will typically have lower energy, but there is an energy barrier that must be overcome in order to transition to the lower energy wetting states. Prior work has demonstrated that transitions can be achieved by vibrating a droplet at its characteristic frequency. However, this is impractical in many applications because every droplet has different vibration frequencies. This project will use frequencies that are characteristic of the surface structure so that they can work on droplets of arbitrary sizes. Specifically, the project will identify the energy required at different frequencies, compare the effectiveness of different vibrational modes, and evaluate the impact of asymmetric vibrations on the fluid wetting. These objectives will be accomplished by both experimental measurements of the pressure to move the wetting front of a liquid on a patterned surface and numerical modeling of the contact lines subjected to vibration.