Roy Pritam Kumar, Takai Yui, Matsubara Rui, Tenjimbayashi Mizuki, Mouterde Timothée
Department of Mechanical Engineering, School of Engineering, The University of Tokyo, Bunkyo, Tokyo 113-8656, Japan.
Research Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan.
Proc Natl Acad Sci U S A. 2025 May 20;122(20):e2500619122. doi: 10.1073/pnas.2500619122. Epub 2025 May 13.
In the insect realm, liquids become traps due to capillary and viscous forces dominant at their scale. Yet, aphids handle the highly viscous honeydew droplets they secrete by coating them with hydrophobic wax powder which maintains an air layer between their body and the liquid. These coated droplets, known as liquid marbles, exhibit low friction and high mobility, enabling manipulation of small liquid volumes which is useful for biomedical analysis where sample volumes are limited, chemistry to reduce chemical waste, or digital microfluidics for large-scale cell culturing and drug testing. For such applications-including exothermic reactions or biological studies typically conducted above room temperature-the ability to carry hot liquid is important but remains unexplored. This article investigates the stability and static friction of hot liquid marbles placed on a substrate cooler by ∆. We show that for large ∆, the core liquid evaporates and condenses within the air layer below the marbles creating liquid bridges resulting in marble rupture on hydrophilic substrates and increased static friction on hydrophobic ones. The temperature difference modifies the nature of static friction: solid friction dominates at small ∆, while at larger ∆, it is replaced by a liquid pinning force caused by the increased liquid bridge density resulting from condensation. Finally, our study provides ways to avoid the rupture and increased static friction of hot liquid marbles due to the bridge formation by increasing the particle size, decreasing the liquid volatility, or using nanostructured superhydrophobic substrates.
在昆虫界，由于毛细力和粘性力在其尺度上占主导地位，液体变成了陷阱。然而，蚜虫通过用疏水性蜡粉覆盖它们所分泌的高粘性蜜露滴来处理这些液滴，从而在它们的身体和液体之间保持一层空气。这些被包裹的液滴，即所谓的液滴弹，表现出低摩擦和高流动性，能够操控少量液体，这对于样本量有限的生物医学分析、减少化学废物的化学实验，或用于大规模细胞培养和药物测试的数字微流控技术都很有用。对于此类应用——包括通常在室温以上进行的放热反应或生物学研究——携带热液体的能力很重要，但仍未得到探索。本文研究了放置在比其温度低∆的基板上的热液滴弹的稳定性和静摩擦力。我们表明，对于较大的∆，核心液体在液滴弹下方的空气层内蒸发和冷凝，形成液桥，导致亲水性基板上的液滴弹破裂，疏水性基板上的静摩擦力增加。温度差改变了静摩擦力的性质：在小∆时，固体摩擦占主导，而在较大∆时，它被由冷凝导致的液桥密度增加所引起的液体钉扎力所取代。最后，我们的研究提供了一些方法，可通过增加颗粒尺寸、降低液体挥发性或使用纳米结构超疏水基板来避免由于桥形成而导致的热液滴弹破裂和静摩擦力增加。
