Department of Chemical and Biomolecular Engineering, Rice University, Houston, TX 77005.
Proc Natl Acad Sci U S A. 2022 Mar 22;119(12):e2117971119. doi: 10.1073/pnas.2117971119. Epub 2022 Mar 14.
Vapor pressure refers to the pressure exerted by the vapor phase in thermodynamic equilibrium with either its liquid or solid phase. An important class of active matter is field-driven colloids. A suspension of dipolar colloids placed in a high-frequency rotating magnetic field undergoes a nonequilibrium phase transition into a dilute and dense phase, akin to liquid–vapor coexistence in a simple fluid. Here, we compute the vapor pressure of this colloidal fluid. The number of particles that exist as the dilute bulk phase versus condensed cluster phases can be directly visualized. An exponential relationship between vapor pressure and effective temperature is determined as a function of applied field strength, analogous to the thermodynamic expression between vapor pressure and temperature found for pure liquids. Additionally, we demonstrate the applicability of Kelvin’s equation to this field-driven system. In principle, this appears to be in conflict with macroscopic thermodynamic assumptions due to the nonequilibrium and discrete nature of this colloidal system. However, the curvature of the vapor–liquid interface provides a mechanical equilibrium characterized by interfacial tension that connects the condensed clusters observed with these active fluids to classical colligative fluid properties.
蒸气压是指与液体或固体相处于热力学平衡的气相所施加的压力。一类重要的活性物质是场驱动胶体。将偶极胶体悬浮液置于高频旋转磁场中，会经历一个非平衡相变，进入稀相和密相，类似于简单流体中的液-气相共存。在这里，我们计算了这种胶体流体的蒸气压。可以直接观察到作为稀相的粒子数量与凝聚团相之间的关系。作为施加场强的函数，蒸气压与有效温度之间呈指数关系，类似于为纯液体找到的蒸气压与温度之间的热力学表达式。此外，我们还证明了开尔文方程在这个场驱动系统中的适用性。原则上，由于这个胶体系统的非平衡和离散性质，这似乎与宏观热力学假设相冲突。然而，蒸-液界面的曲率提供了一个由界面张力表征的力学平衡，将观察到的凝聚团与这些活性流体连接到经典的依数性质上。
