Seeding effects in phase transitions quenched far from of equilibrium

相变及其对称性破缺机制的研究一直是量子力学领域的一个热门课题。统计物理学的一个巨大成就是根据对称性和相应序参量的对称性将不同的系统划分为普适类。例如，3D和2D系统中的融化场景非常不平等。成功的统计描述的先决条件是，系统处于热力学平衡。一个著名的例子是系统失去平衡的玻璃化转变，另一个例子是具有极大冷却速率的结晶，其中系统远离热力学平衡。对于2D系统，我们能够表明，平衡和不平衡的冻结机制是完全不同的。在这里，我们想量化结晶远离平衡在微观水平上，而在温度淬火过程中提供的成核seed.该系统由超顺磁性胶体粒子被限制在一个绝对平坦的液体/空气界面构成一个2D单层。由于粒子的动力学是足够慢的数字图像处理解决的完整的相空间信息是可用的，可以比较的理论模型。使用外部磁场，可以改变粒子的排斥相互作用与热能之间的比率，从而控制作为时间的函数的系统温度。这种方法可以使系统在比固有时间尺度短10-4倍的时间尺度上冷却--这在我们的实验中是一种独特的可行性。由于如我们先前所示，平衡相变不能用经典成核理论描述，我们想研究局域序参数波动的影响。因此，我们希望在淬火期间使用全息光镊施加种子（小的晶体结构或壁）。这种晶种在淬火过程中的效果（我们避免将其命名为异质成核）将在该项目中通过改变不同的参数，包括种子的大小及其几何形状，淬火速率和深度进行彻底研究。最后，我们将把结果与布朗动力学模拟、动态密度泛函理论和相场理论计算的预测进行比较，以深入了解这种自组织的非平衡过程

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