沉降对带电胶体粒子结晶过程的影响

采用密度匹配法(重水与水按一定比例混合)，以及反射光谱法，研究了重力沉降作用对直径为98 nm的带电胶体粒子结晶过程的影响. 结果表明，重力沉降在晶体生长的初期提高了晶体生长速率，而后期降低了晶体生长速率，这是由于在晶体生长初期，沉降作用可使更多的粒子结合到晶体结构中，而当晶体尺寸进一步增加，其沉降速率也相应地增大，晶相与液相间的摩擦阻力导致一部分颗粒从胶体晶体上脱落. 总的来说，重力沉降在初期加剧了晶体的生长，后期阻碍了晶体的生长. 另外，在微重力环境下形成的胶体晶体比在重力环境下形成的胶体晶体更加完整紧密.

Influence of Sedimentation on Crystallization of Charged Colloidal Particles

The method of density matching between the solid and liquid phases is often adopted to effectively eliminate the effect of sedimentation of suspensions on dynamic behavior of a colloidal system. Experiments on crystallization of charged colloidal microspheres with di-ameter of 98 nm dispersed in density-matched and -unmatched media (mixtures of H₂O and D₂O in proper proportion) are compared to examine the influence of sedimentation. Reflection spectra of colloidal suspensions were used to monitor the crystallization process. Results showed that the crystal size of the density-unmatched (namely, in the presence of sedimentation) sample grew faster than that of the density-matched (in the absence of sedi-mentation) case at the initial stage of the crystallization, and then the latter overtook and outstripped the former. To explain these observations, we assume that in the settling of crystals sedimentation facilitates result in more particles getting into the crystal structures. However, as the crystals increase to varying sizes, the settling velocities become large and hydrodynamic friction strips off some particles from the delicate crystal structures. Overall, the sedimentation appears to accelerate the crystal size growth initially and then retard the growth. In addition, the crystal structures formed under microgravity were more closely packed than that in normal gravity.