Contact Angle Hysteresis on Curved Surfaces

液滴和固体之间的接触出现在许多日常环境中，例如汽车上的雨滴，植物叶子上的露水凝结，或昆虫在池塘上行走。为了理解或控制这些系统和相关系统的行为，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）之间存在界面，该界面与固体物体的表面相遇。流体界面和固体表面之间的角度（“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，当流体界面在干燥表面上前进时，接触角具有一致的值，而当界面后退（沿另一方向移动）时，接触角具有较小的值。这种接触角的变化性，称为滞后，决定了关键的性质，如液滴是否从表面滑落或保持捕获。接触角滞后没有通用理论，但通常认为是所用材料的固有特性。该项目研究了接触角滞后也随流体界面形状而变化的新发现。该项目是重要的，因为了解接触角滞后的几何作用将提供更广泛的见解，我们目前缺乏的现象。最终，该项目的成果将有助于改进工艺流程，如用流体涂覆纤维，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浮在液体表面上来组装电子元件。该项目将通过在前进和后退流体的条件下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。实验集中在毫米级的对象，使界面可以直接可视化。从已知的界面形状，表面能将计算数值，并在可能的情况下，分析。这些结果将允许测试接触角滞后的拟议机制。除了产生新的基础科学，拟议的研究将支持当地中学生的教育项目和马萨诸塞州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，并已被认为是值得支持的，通过评估使用基金会的智力价值和更广泛的影响审查标准。

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.