Joshipura Ishan D, Persson K Alex, Truong Vi Khanh, Oh Ji-Hyun, Kong Minsik, Vong Man Hou, Ni Chujun, Alsafatwi Mohanad, Parekh Dishit P, Zhao Hong, Dickey Michael D
Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, 27695 North Carolina, United States.
Materials Engineering Division, Lawrence Livermore National Laboratory, Livermore, 27695 California, United States.
Langmuir. 2021 Sep 21;37(37):10914-10923. doi: 10.1021/acs.langmuir.1c01173. Epub 2021 Sep 7.
This work establishes that static contact angles for gallium-based liquid metals have no utility despite the continued and common use of such angles in the literature. In the presence of oxygen, these metals rapidly form a thin (∼1-3 nm) surface oxide "skin" that adheres to many surfaces and mechanically impedes its flow. This property is problematic for contact angle measurements, which presume the ability of liquids to flow freely to adopt shapes that minimize the interfacial energy. We show here that advancing angles for a metal are always high (>140°)-even on substrates to which it adheres-because the solid native oxide must rupture in tension to advance the contact line. The advancing angle for the metal depends subtly on the substrate surface chemistry but does not vary strongly with hydrophobicity of the substrate. During receding measurements, the metal droplet initially sags as the liquid withdraws from the "sac" formed by the skin and thus the contact area with the substrate initially increases despite its volumetric recession. The oxide pins at the perimeter of the deflated "sac" on all the surfaces are tested, except for certain rough surfaces. With additional withdrawal of the liquid metal, the pinned angle gets smaller until eventually the oxide "sac" collapses. Thus, static contact angles can be manipulated mechanically from 0° to >140° due to hysteresis and are therefore uninformative. We also provide recommendations and best practices for wetting experiments, which may find use in applications that use these alloys such as soft electronics, composites, and microfluidics.
这项工作表明，尽管镓基液态金属的静态接触角在文献中仍被持续且普遍使用，但其实并无实际用途。在有氧气存在的情况下，这些金属会迅速形成一层薄（约1 - 3纳米）的表面氧化“皮”，该氧化皮会附着在许多表面上，并在机械上阻碍其流动。这种特性对于接触角测量来说是个问题，因为接触角测量假定液体能够自由流动以采用使界面能最小化的形状。我们在此表明，金属的前进接触角总是很高（>140°）——即使在它附着的基底上也是如此——因为固态原生氧化物必须在张力作用下破裂才能推进接触线。金属的前进接触角在一定程度上取决于基底表面化学性质，但不会随基底的疏水性有很大变化。在后退测量过程中，当液体从由氧化皮形成的“囊”中抽出时，金属液滴最初会下垂，因此尽管其体积减小，但与基底的接触面积最初会增加。除了某些粗糙表面外，对所有表面上瘪掉的“囊”周边的氧化皮固定情况进行了测试。随着液态金属的进一步抽出，固定角会变小，直到最终氧化“囊”塌陷。因此，由于滞后现象，静态接触角可以从0°机械调节到>140°，所以它并无参考价值。我们还提供了润湿性实验的建议和最佳实践方法，这些方法可能会在使用这些合金的应用中有所帮助，比如软电子学、复合材料和微流体学等领域。
