标题： 由强聚焦激光束引起的相变振荡 显示英文标题

标题： Phase transition oscillations induced by a strongly focused laser beam

摘要： 我们在这里报告观察到一种令人惊讶的现象，即在二元混合物PMMA/3-辛酮中，当它被强聚焦的红外激光束照射时出现的振荡相变。PMMA/3-辛酮具有一个UCST（上临界溶解温度），在温度Tc = 306.6 K和体积分数$\phi$c = 12.8% [Crauste等，ArXiv 1310.6720, 2012]处存在临界点。这种振荡现象是由于热泳和电致收缩效应以及非线性扩散引起的。我们分析了这些振荡，并提出一个简单的模型，该模型包含产生振荡行为所需的最小要素。二元混合物中的相变仍然是一个广泛研究的主题，特别是在临界点附近，可能出现多种有趣且尚未完全理解的现象，其中我们提到临界卡西米尔力[2]、[3]、限制效应[4]、[5]以及淬火后的非平衡动力学。通过外部场扰动二元混合物也是一个重要且最近的研究领域[6]。例如，激光可以在分相二元混合物中引发有趣的现象，因为辐射压力可以变形两相之间的界面，并可用于测量界面张力[7]。根据二元混合物的性质，激光照射也可能导致混合或分相转变。在参考文献[8]中，聚焦的红外激光光加热初始处于均相状态的介质，并导致LCST（下临界溶解温度）系统发生分离。激光束中的辐射压力梯度也促进了聚合物的聚集，从而产生相变。局部加热可能会引起热泳力，使二元混合物的一种组分向激光束吸引[9]。其他力如电致收缩也可能参与其中[10]。在这封信中，我们报告了一种新现象，即由红外激光束在UCST（上临界溶解温度）二元混合物的非均质区域恒定照射引起的振荡相变。相变中的振荡现象已经在缓慢冷却的UCST中被报道过[11]、[12]，但据我们所知，从未由静止激光照射引发。在描述我们的实验装置后，我们将展示结果。然后我们将使用一个非常简化的模型，该模型包含引发这种振荡现象的主要必要物理要素。 显示英文摘要

摘要： We report here the observation of a surprising phenomenon consisting in a oscillating phase transition which appears in a binary mixture, PMMA/3-octanone, when this is enlightened by a strongly focused infrared laser beam. PMMA/3-octanone has a UCST (Upper Critical Solution Temperature) which presents a critical point at temperature Tc = 306.6 K and volume fraction $\phi$c = 12.8 % [Crauste et al., ArXiv 1310.6720, 2012]. This oscillatory phenomenon appears because of thermophoretic and electrostriction effects and non-linear diffusion. We analyze these oscillations and we propose a simple model which includes the minimal ingredients to produce the oscillatory behavior. Phase transitions in binary mixtures are still a widely studied subject, specifically near the critical point where several interesting and not completely understood phenomena may appear, among them we recall the critical Casimir forces [2],[3], confinement effects [4], [5] and out-of-equilibrium dynamics after a quench. The perturbation of the binary mixtures by mean of external fields is also an important and recent field of investigation [6]. For example, a laser can induce interesting phenomena in demixing binary mixtures because the radiation pressure can deform the interface between the two phases and it can be used to measure the interface tension [7]. Depending on the nature of the binary mixtures, laser illumination can also lead to a mixing or demixing transition. In ref.[8], focused infrared laser light heats the medium initially in the homogeneous phase and causes a separation in the LCST (Low Critical Solution Temperature) system. The radiation pressure gradients in a laser beam also contribute in the aggregation of polymers , thus producing a phase transition. The local heating may induce thermophoretic forces which attract towards the laser beam one of the binary-mixture components [9]. Other forces like electrostriction can also be involved [10]. In this letter, we report a new phenomenon, which consists in an oscillating phase transition induced by a constant illumination from an infrared laser beam in the heterogeneous region of an UCST (Upper Critical Solution Temperature) binary mixture. Oscillation phenomena in phase transition have already been reported in slow cooling UCST [11],[12] but as far as we know, never induced by a stationary laser illumination. After describing our experimental setup , we will present the results. Then we will use a very simplified model which contains the main necessary physical ingredients to induce this oscillation phenomenon.