Contact Angle Hysteresis on Curved Surfaces

液滴和固体之间的接触在许多日常环境中都会发生，比如汽车上的雨滴，植物叶子上的露水凝结，或者昆虫在水塘里行走。要了解或控制这些和相关系统的行为，我们必须首先了解流体表面的形状和它们可以施加的力。在所有这些系统中，两种流体(例如空气和水)之间有一个界面，它与固体物体的表面相遇。流体界面和固体表面之间的夹角(“接触角”)是决定界面形状和流体施加的力的关键参数。实验表明，当流体界面前进穿过干燥表面时，接触角有一个恒定值，当界面后退(向相反方向移动)时，接触角有一个较小的值。接触角的这种可变性，即所谓的滞后，决定了关键属性，如液滴是从表面滑落还是仍被困住。接触角滞后没有一般的理论，但它通常被认为是所使用的材料的固有特性。这个项目研究了一个新的发现，接触角滞后也随着流体界面的形状而变化。这个项目很重要，因为了解几何在接触角滞后中的作用将使我们对目前缺乏的现象有更广泛的见解。最终，该项目的成果将有助于改进工艺过程，如在纤维上涂覆流体，在食品或石油回收应用中用颗粒稳定油滴，甚至通过将电子元件漂浮在液体表面来组装它们。该项目将通过在流体进退条件下通过直接观察来测量接触线形状和接触角。固体表面的化学处理将有所不同，并将使用一系列流体。实验集中在毫米级物体上，这样界面就可以直接可视化。根据已知的界面形状，表面能量将通过数值计算，并在可能的情况下，以解析的方式计算。这些结果将允许对所提出的接触角滞后机制进行测试。除了产生新的基础科学，拟议的研究还将支持当地中学生的教育项目以及马萨诸塞大学阿默斯特大学本科生和研究生的职业培训。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Contact between liquid droplets and solids arise in many everyday settings such as rain droplets on a car, condensation of dew on plant leaves, or insects walking on water ponds. To understand or control the behavior of these and related systems, we must first understand the shape of the fluid surfaces and the forces that they can apply. In all of these systems, there is an interface between two fluids (air and water, for example), which meets the surface of a solid object. The angle between the fluid interface and the solid surface (the "contact angle") is the key parameter that determines the interface shape and the forces applied by the fluids. Experiments show that the contact angle has a consistent value when a fluid interface advances across a dry surface, and a smaller value when the interface recedes (moves in the other direction). This variability of the contact angle, known as hysteresis, determines key properties such as whether droplets slide off a surface or remain trapped. There is no general theory for contact-angle hysteresis, but it is typically thought to be an inherent property of the materials being used. This project investigates a new finding that contact angle hysteresis also varies with the shape of the fluid interface. The project is important because understanding the role of geometry in contact-angle hysteresis will give broader insights into the phenomenon that we currently lack. Ultimately, the results of this project will contribute to improved technological processes such as coating of fibers with fluids, stabilizing oil droplets with particles in food or oil-recovery applications, and even assembling electronic components by floating them on a liquid surface.This project will provide measurements of the of the contact-line shape and the contact angle by direct observation under conditions of advancing and receding fluids. Chemical treatment of the solid surface will be varied and a range of fluids will be used. Experiments focus on millimeter-scale objects so that the interface can be directly visualized. From the known interface shapes, the surface energies will be calculated numerically and, where possible, analytically. These results will allow testing of proposed mechanisms of contact-angle hysteresis. In addition to generating new basic science, the proposed research will support educational projects for local middle-school students and career training for undergraduate and graduate students at the University of Massachusetts Amherst.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.