Liquid Metal Contact Angle Measurement in Vacuum

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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.