Tito Nicholas B, Milner Scott T, Lipson Jane E G
Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, UK.
Soft Matter. 2015 Oct 21;11(39):7792-801. doi: 10.1039/c5sm01701g. Epub 2015 Aug 27.
The diffusion of mobility in bulk and thin film fluids near their glass transition is examined with a kinetic lattice model, and compared to recent experiments on bulk liquids and vapor-deposited thin film glasses. The "limited mobility" (LM) lattice model exhibits dynamic heterogeneity of mobility when the fluid is near its kinetic arrest transition; a finite-parameter second-order critical point in the LM model bearing strong resemblance to the glass transition in real fluids. The spatial heterogeneity of mobility near kinetic arrest leads to dynamics that violate the Stokes-Einstein relation. To make connections with experiment, LM model simulations of self-diffusion constants in fluids near kinetic arrest are compared to those in two organic glass-formers. In addition, simulations of mobility in films that have been temperature-jumped above kinetic arrest (starting from an arrested state) are carried out. The films develop a "front" of mobility at their free surface that progresses into the film interior at a constant rate, thereby mobilising the entire film to fluidity. The velocity of the front scales with the self-diffusion constant for analogous bulk systems-an observation consistent with experiments on vapor-deposited molecular thin films.
利用动力学晶格模型研究了本体和薄膜流体在其玻璃化转变附近的迁移率扩散，并与近期关于本体液体和气相沉积薄膜玻璃的实验进行了比较。“有限迁移率”（LM）晶格模型在流体接近其动力学阻滞转变时表现出迁移率的动态不均匀性；LM模型中的一个有限参数二阶临界点与实际流体中的玻璃化转变非常相似。动力学阻滞附近迁移率的空间不均匀性导致动力学违反斯托克斯-爱因斯坦关系。为了与实验建立联系，将动力学阻滞附近流体中自扩散常数的LM模型模拟结果与两种有机玻璃形成体中的结果进行了比较。此外，还对从阻滞状态开始温度跃升至高于动力学阻滞温度的薄膜中的迁移率进行了模拟。薄膜在其自由表面形成一个迁移率“前沿”，该前沿以恒定速率向薄膜内部推进，从而使整个薄膜变为流体状态。前沿速度与类似本体系统的自扩散常数成比例——这一观察结果与气相沉积分子薄膜的实验一致。
