Binder Kurt, Virnau Peter, Statt Antonia
Institut für Physik, Johannes Gutenberg-Universität Mainz, Staudinger Weg 9, 55128 Mainz, Germany.
Graduate School of Excellence Material Science in Mainz, Staudinger Weg 9, 55128 Mainz, Germany.
J Chem Phys. 2014 Oct 14;141(14):140901. doi: 10.1063/1.4896943.
In many colloidal suspensions, the micrometer-sized particles behave like hard spheres, but when non-adsorbing polymers are added to the solution a depletion attraction (of entropic origin) is created. Since 60 years the Asakura-Oosawa model, which simply describes the polymers as ideal soft spheres, is an archetypical description for the statistical thermodynamics of such systems, accounting for many features of real colloid-polymer mixtures very well. While the fugacity of the polymers (which controls their concentration in the solution) plays a role like inverse temperature, the size ratio of polymer versus colloid radii acts as a control parameter to modify the phase diagram: when this ratio is large enough, a vapor-liquid like phase separation occurs at low enough colloid packing fractions, up to a triple point where a liquid-solid two-phase coexistence region takes over. For smaller size ratios, the critical point of the phase separation and the triple point merge, resulting in a single two-phase coexistence region between fluid and crystalline phases (of "inverted swan neck"-topology, with possibly a hidden metastable phase separation). Furthermore, liquid-crystalline ordering may be found if colloidal particles of non-spherical shape (e.g., rod like) are considered. Also interactions of the particles with solid surfaces should be tunable (e.g., walls coated by polymer brushes), and interfacial phenomena are particularly interesting experimentally, since fluctuations can be studied in the microscope on all length scales, down to the particle level. Due to its simplicity this model has become a workhorse for both analytical theory and computer simulation. Recently, generalizations addressing dynamic phenomena (phase separation, crystal nucleation, etc.) have become the focus of studies.
在许多胶体悬浮液中，微米级的颗粒表现得像硬球，但当向溶液中加入非吸附性聚合物时，就会产生一种（源于熵的）排空吸引力。60年来，浅仓-大泽模型将聚合物简单描述为理想的软球，是此类系统统计热力学的典型描述，能很好地解释实际胶体-聚合物混合物的许多特征。聚合物的逸度（控制其在溶液中的浓度）起着类似逆温度的作用，而聚合物与胶体半径的尺寸比则作为一个控制参数来改变相图：当这个比值足够大时，在足够低的胶体填充分数下会发生类似气-液的相分离，直至出现一个三相点，此时液-固两相共存区域开始出现。对于较小的尺寸比，相分离的临界点和三相点合并，导致流体相和结晶相之间出现单一的两相共存区域（具有“倒天鹅颈”拓扑结构，可能存在隐藏的亚稳相分离）。此外，如果考虑非球形的胶体颗粒（例如棒状），可能会发现液晶有序性。颗粒与固体表面的相互作用也应是可调的（例如，由聚合物刷包覆的壁），而且界面现象在实验上特别有趣，因为可以在显微镜下研究从颗粒尺度到所有长度尺度的涨落。由于其简单性，这个模型已成为解析理论和计算机模拟的得力工具。最近，针对动态现象（相分离、晶体成核等）的推广已成为研究的焦点。
