Seeding effects in phase transitions quenched far from of equilibrium

长期以来，相变及其对称性破缺机制的研究一直是物理学中的一个重要研究课题。根据系统的对称性和各序参数的对称性，将完全不同的系统划分为普适类，这是统计物理学的一项巨大成就。例如，3D和2D系统中的融化场景非常不同。成功的统计描述的一个前提是，系统处于热力学平衡。系统失去平衡的一个著名例子是玻璃化转变，另一个例子是冷却速度极快的结晶，其中系统远远超出热力学平衡。对于2D体系，我们能够证明平衡冻结和非平衡冻结的机制是完全不同的。在这里，我们希望在微观水平上量化远离平衡的结晶，同时在温度淬火过程中提供成核种子。该体系由限制在绝对平坦的液/气界面上的超顺磁性胶体颗粒组成，构成了2D单分子膜。由于粒子的动力学足够慢，可以通过数字图像处理来分辨，因此可以获得完整的相空间信息，并可以与理论模型进行比较。利用外加磁场，粒子间的排斥相互作用与热能之间的比率可以改变，从而控制系统温度作为时间的函数。通过这种方式，系统可以在比固有时间短10-4倍的时间尺度上冷却，这在我们的实验中是唯一可行的。由于非平衡相变不能用经典成核理论来描述，正如我们前面所表明的，我们想要研究局域有序参数的涨落的影响。因此，我们希望在猝灭期间使用全息光学镊子来施加种子(小的晶体结构或墙)。在这个项目中，将通过改变种子的大小及其几何、淬火速度和深度等不同参数来彻底研究这种晶种在淬火(我们避免将其命名为异质成核)过程中的效果。最后，我们将把结果与布朗动态模拟、动态密度泛函理论和相场理论的预测进行比较，以更深入地了解这种脱离平衡的自组织过程

The investigation of phase transitions and the corresponding symmetry breaking mechanism is a fascinating research topic in physic since long. It has been an enormous achievement in statistical physics to classify quite different systems into universality classes depending on their symmetries and the symmetries of the respective order parameters. For Example, the melting scenarios in 3D and 2D systems are very unequal. A prerequisite for the successful statistical description was, that the systems are in thermodynamic equilibrium. One famos example for a system falling out of equilibrium is the glass transition yet another is crystallization with extremely large cooling rates where systems are far out of thermodynamic equilibrium. For 2D system we were able to show, that the mechanism of freezing in equilibrium and out of equilibrium are quite different. Here we would like quantify crystallization far from equilibrium on a microscopic level, while a nucleation seed is offered during a temperature quench.The system consists of super-paramagnetic colloidal particles confined at an absolutely flat liquid/air interface constituting a 2D monolayer. Since the dynamics of the particles is slow enough to be resolved by digital image processing the full phase-space information is available and can be compared to theoretical models. Using an outer magnetic field, the ratio between repulsive interaction of the particles and thermal energy can be varied, controlling the system temperature as function of time. This way the system can be cooled on time scales 10-4-times shorter compared to intrinsic time scales - a unique feasibility in our experiment.Since the out of equilibrium phase transition can not be described with classical nucleation theory as we showed previously we want to investigate the influence of fluctuations of the local order parameters. Therefore we want to impose a seed (small crystalline structures or walls) using holographic optical tweezers during the time of quench. The effect of such a crystalline seed during the quench (where we avoid to name it heterogeneous nucleation) will be studied thoroughly in this project by varying different parameters including the size of the seed and its geometry, quench rate and depth. We will finally compare the results with predictions of Brownian dynamic simulations, dynamic density functional theory, and phase field theory calculations to get deeper insight in this out of equilibrium process of self organization