标题： 胶体在 bull's-eye 图案上方的临界卡西米尔升腾现象 显示英文标题

标题： Critical Casimir levitation of colloids above a bull's-eye pattern

摘要： 临界卡西米尔力出现在近临界流体中的粒子或表面之间，这种力的符号由表面特性决定，并且其强度可以通过微小的温度变化进行调节。 在这里，我们展示了如何利用这种力来捕获胶体粒子，并使其悬浮在具有“靶心”图案的基底上方，该图案的环具有与基底其他部分相反的表面特性。 通过德热纳近似和平均场计算，我们发现球形胶体在这样的图案表面上表现出丰富的行为，包括向环沉降以及在环上方（环悬浮）或在靶心中心上方（点悬浮）悬浮。 在德热纳近似下，我们计算了点悬浮的悬浮图，显示了捕获势阱的深度以及胶体悬浮的高度，两者都取决于图案特性、胶体大小和溶液温度。 我们的计算表明，如果系统远离临界点，与点悬浮相关的参数空间会缩小，而令人惊讶的是，捕获力会变得更强。 我们讨论了临界卡西米尔悬浮在按尺寸分选胶体以及确定热力学距离临界点的应用。 我们的结果表明，临界卡西米尔力为控制图案表面上胶体粒子的行为提供了丰富的机会。 显示英文摘要

摘要： Critical Casimir forces emerge among particles or surfaces immersed in a near-critical fluid, with the sign of the force determined by surface properties and with its strength tunable by minute temperature changes. Here, we show how such forces can be used to trap a colloidal particle and levitate it above a substrate with a bull's-eye pattern consisting of a ring with surface properties opposite to the rest of the substrate. Using the Derjaguin approximation and mean-field calculations, we find a rich behavior of spherical colloids at such a patterned surface, including sedimentation towards the ring and levitation above the ring (ring levitation) or above the bull's-eye's center (point levitation). Within the Derjaguin approximation, we calculate a levitation diagram for point levitation showing the depth of the trapping potential and the height at which the colloid levitates, both depending on the pattern properties, the colloid size, and the solution temperature. Our calculations reveal that the parameter space associated with point levitation shrinks if the system is driven away from a critical point, while, surprisingly, the trapping force becomes stronger. We discuss the application of critical Casimir levitation for sorting colloids by size and for determining the thermodynamic distance to criticality. Our results show that critical Casimir forces provide rich opportunities for controlling the behavior of colloidal particles at patterned surfaces.