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，这对于包括生物，医学和MEMS/NEMS在内的广泛应用和设备非常重要。因此，在宏观尺度和上微观尺度上，已经开发了几种静态和动态接触角测量技术。此外，在纳米尺度和真空环境下的接触角测量也取得了一些进展。然而，由于大的蒸发速率和难以操作和处理小液滴，没有可靠的方法在微纳米尺度上测量接触角。这导致缺乏可靠的实验验证的单一和少数粗糙度接触力学，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不计的蒸气压，是制造纳米级液滴的有希望的候选者，但在环境条件下，它们在一瞬间就会形成氧化层。这一层阻止了真正的液固接触，并抑制了接触角的测量。然而，当在真空中生产时（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，如果灵巧地操作，可以用来表征真空条件下不同质地表面的润湿性，即不受环境气氛的影响。本文旨在进一步开发和优化一种新型的接触角测量方法，该方法使用无氧化物纳米级液态金属液滴，并应用于扫描电镜真空室的高压条件下，为接触角值的提取提供视觉反馈。然后，该方法将用于在10微米以下迄今无法达到的长度尺度上系统地研究少量和单个粗糙接触力学。结果将用于验证现有的理论和模拟，从而加深对这种尺寸尺度上的润湿和液固接触力学的一般理解。此外，所开发的方法还将通过应用案例进行验证，以证明其在研究材料润湿性方面的可用性，这些材料来自高度局部化且迄今为止难以研究的（由于规模小）材料领域，如靶向药物输送和微塑料。

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.