摘要:
对于带高电荷的胶体粒子形成的胶体晶体,用现有的剪切模量与粒子间相互作用的理论模型去计算剪切模量时,得到的结果要比实验测量值大很多,而根据此理论模型由剪切模量测量值拟合得到的粒子表面有效电荷也比实验测量的表面有效电荷小很多.这个问题一直没有合理的解释.分析认为这是由于理论模型基于理想晶体,并没有考虑到实际晶体中存在孔隙等缺陷会造成材料机械性能的下降.针对该因素,本文对已有的理论模型进行了修正,引入了孔隙造成的影响,同时使用Sogami-Ise相互作用势代替原来常用的Yukawa作用势.研究中用扭转共振法测量了带电粒子胶体晶体的剪切模量,比较了分别根据修正前后的关系模型由所测剪切模量拟合的有效电荷(弹性有效电荷),证实修正后的理论模型更合理,与使用Alexander等的方法得到的归一化电荷相比,一致性有了明显提高.
关键词:
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有效电荷
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剪切模量
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孔隙
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胶体晶体
Abstract:
According to the existing shear modulus-pair potential relationship model for colloidal crystal comprised of highly charged colloidal particles, the calculated shear moduli of colloidal crystals are much larger than the measured values by the torsional resonance spectroscopy (TRS). Moreover, by using the relationship model, the effective surface charge of colloidal particles, obtained by fitting values of shear moduli measured by TRS (effective elasticity charge), is smaller than that obtained through the experimental method of conductivity-number density relationship (effectively transported charge). So far there has been no practical explanation to this discrepancy. Our analysis shows that this discrepancy is because the existing relationship model is for the perfect crystals and does not include the defects such as voids which can result in the decrease of mechanical properties of materials. The existing shear modulus-pair potential model will be improved by introducing the effect of voids, which is inspired from the Gibson-Ashby model in the study of cellular solid. The Yukawa potential, which considers Coulomb repulsions between colloidal particles and is usually used in the model expressions, will be substituted by Sogami-Ise potential, which considers a long-range attraction in addition to that Coulomb repulsions and accepts the existence of voids inside the colloidal crystals. For five different kinds of highly charged colloidal particles, the shear moduli with different volume fractions are measured by TRS. Then the fitted effective surface charges using the original and improved model respectively are compared with each other. It can be concluded that the effective elastic charge obtained by the improved model is more suitable and much closer to the renormalized charge obtained from Alexander's method. It is also clear that neither the effectively transported charge nor the Alexander's renormalized charge can be used to evaluate the shear moduli of colloidal crystals with voids inside. These results can also let us further understand and use the effective surface charge in the colloid studies.
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