Liquid Metal Contact Angle Measurement in Vacuum

接触角测量已经成为表征表面润湿性和能量的标准方法，这对包括生物、医学和MEMS/NEMS在内的广泛应用和设备非常重要。因此，已经开发了几种技术来测量宏观尺度和上微观尺度上的静态和动态接触角。此外，人们一直在努力测量纳米尺度和真空环境中的接触角。然而，由于蒸发速率大，难以操纵和处理小液滴，目前还没有在微米和纳米尺度上可靠测量接触角的方法。这导致缺乏可靠的单一和少数粗糙接触力学的实验验证，以验证现有的理论和模拟结果。现有的技术仅限于高度专业化的应用，不允许进行系统的研究。另一方面，镓基液态金属合金由于其可忽略的蒸汽压而有望产生纳米级液滴，但在环境条件下，它们在一秒内形成一层氧化层。这一层防止了真正的液-固接触，并阻止了接触角的测量。然而，当这些液态金属液滴在真空中产生时(例如通过电迁移)，这些液态金属液滴可以保持几个小时的无氧化物状态，如果操作灵活，可以在真空条件下表征不同纹理的表面的润湿性，即不受环境大气的影响。本建议旨在进一步开发和优化一种新的接触角测量方法，该方法使用无氧化物纳米液态金属液滴，应用于扫描电子显微镜真空室内的高压条件下，为接触角值的提取提供视觉反馈。然后，这种方法将被用来在10微米以下的长度尺度上对少数和单个粗糙面接触力学进行系统的研究。研究结果将被用来验证现有的理论和模拟，从而加深对润湿和液固接触力学在这种尺度上的一般理解。此外，开发的方法还将通过应用案例进行验证，以证明其在研究高度热门和迄今难以研究(由于尺寸小)的材料领域(如靶向给药和微塑料)材料的润湿性方面的可用性。

Contact angle measurement has become a standard approach to characterize the wettability and energy of a surface, which is important for a wide range of applications and devices, including biology, medicine, and MEMS/NEMS. Thus, several techniques have been developed for static as well as dynamic contact angle measurements on the macroscale and upper microscale. Moreover, there have been efforts to measure contact angles on the nanoscale and in vacuum environment. However, due to large evaporation rates and difficulty to manipulate and handle small droplets, there is no methodology for reliable contact angle measurements on the micro- and nanoscale. This leads to the lack of reliable experimental validation of single and few asperity contact mechanics in order to verify available theoretical and simulation results. The existing techniques are limited to highly specialized applications, which do not allow for a systematic study.On the other hand, gallium-based liquid metal alloys are promising candidates for creation of nanoscale droplets due to their negligible vapor pressure, but they suffer from an oxide layer forming within a split of a second under ambient conditions. This layer prevents true liquid-to-solid contact and inhibits contact angle measurement. However, when produced in vacuum (e.g. via electromigration), those liquid metal droplets can maintain an oxide free state for several hours and, if dexterously manipulated, can be used to characterize the wettability of surfaces with different texture under vacuum conditions, i.e. without the influence of the ambient atmosphere.This proposal aims at further development and optimization of a novel contact angle measurement method using oxide free nanoscopic liquid metal droplets and applied in HV conditions inside the vacuum chamber of an SEM that provides visual feedback for extraction of contact angle values. This method will then be utilized for a systematic investigation of few- and single asperity contact mechanics on a hitherto inaccessible length scale below 10 micrometers. The results will be used to verify existing theory and simulations and thus deepen the general understanding of wetting and liquid-solid contact mechanics on this size scale. Furthermore, the developed method will also be validated by application cases to prove its usability for studying the wettability properties of materials from highly topical and hitherto difficult to investigate (due to the small size scale) material fields such as targeted drug delivery and microplastics.