The interplay of sedimentation and crystallization in hard-sphere suspensions
The interplay of sedimentation and crystallization in hard-sphere suspensions
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DOI：
10.1039/c3sm50980j
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发表时间：
2013-07
期刊：
影响因子：
3.4
通讯作者：
J. Russo;A. Maggs;D. Bonn;Hajime Tanaka
中科院分区：
文献类型：
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作者：
J. Russo;A. Maggs;D. Bonn;Hajime Tanaka
通过布朗动力学模拟研究了胶体硬球模型中沉淀作用对晶体成核的影响。我们引入了两个作用在胶体流体上的外场：均匀引力场(体力)和通过钉扎一层平衡粒子而产生的表面场(粗糙壁)。我们发现，由于动力学的减慢，晶体在壁面附近的形核受到抑制，并且这种影响的空间范围由键取向级数的静态长度尺度决定。当距离壁面的距离大于此长度尺度时，由于沉积过程导致靠近底部的体积分数较高或过冷度较高，因此形核率大大提高。成核阶段类似于均匀的情况，原子核平均为球形，晶面在空间中随机取向。相反，生长阶段受引力场引入的对称性破缺的影响很大，密度梯度减缓了附着速率，这反过来又导致原子核在横向上生长得更快。我们的发现表明，高密度区晶体形核率的增加可能是硬球形核率与实验结果相差很大的原因，并指出以前的实验中的引力效应是不可忽略的。
We study crystal nucleation under the influence of sedimentation in a model of colloidal hard spheres via Brownian dynamics simulations. We introduce two external fields acting on the colloidal fluid: a uniform gravitational field (body force), and a surface field imposed by pinning a layer of equilibrium particles (rough wall). We show that crystal nucleation is suppressed in proximity of the wall due to the slowing down of the dynamics, and that the spatial range of this effect is governed by the static length scale of bond orientational order. For distances from the wall larger than this length scale, the nucleation rate is greatly enhanced by the process of sedimentation, since it leads to a higher volume fraction, or a higher degree of supercooling, near the bottom. The nucleation stage is similar to the homogeneous case, with nuclei being on average spherical and having crystalline planes randomly oriented in space. The growth stage is instead greatly affected by the symmetry breaking introduced by the gravitation field, with a slowing down of the attachment rate due to density gradients, which in turn cause nuclei to grow faster laterally. Our findings suggest that the increase of crystal nucleation in higher density regions might be the cause of the large discrepancy in the crystal nucleation rate of hard spheres between experiments and simulations, on noting that the gravitational effects in previous experiments are not negligible.