Gillies Graeme, Prestidge Clive A
Ian Wark Research Institute, The ARC Special Research Centre for Particle and Material Interfaces, University of South Australia, Mawson Lakes, South Australia, 5095, Australia.
Langmuir. 2005 Dec 20;21(26):12342-7. doi: 10.1021/la0515120.
The repulsive forces between a glass sphere and immobilized colloidal droplets of poly(dimethylsiloxane) (PDMS) (with various levels of internal cross-linking) have been determined in aqueous solution using colloid probe atomic force microscopy. On initial surface approach, droplet deformation is negligible and interaction forces resemble those expected for electrical double layer interaction of rigid spheres. Upon further approach, droplet flattening results in forces that deviate below rigid body electrical double layer interaction. The extent of droplet deformation has been determined in terms of the deviation from hard-sphere interaction. Droplet deformability is strongly dependent on the droplet cross-linking level and hence controlled by some combination of the bulk rheological and interfacial properties of the droplets. Droplet nano-rheology has been determined from the extent of force curve hysteresis. For liquidlike droplets, with low levels of cross-linking, no force curve hysteresis is observed and the elastic deformation may be described by a single spring constant, which is controlled by the interfacial properties. For highly cross-linked droplets, the extent of deformation is controlled by the droplet's bulk rheology rather than the interfacial properties. Upon retraction of the surfaces, force curve hysteresis is observed and is due to the viscoelastic response of the PDMS. The extent of hysteresis is dependent on the rate of approach/retraction and the loading force and has been theoretically analyzed to determine nano-rheological parameters that describe droplet relaxation processes. Elastic moduli and relaxation times of the PDMS droplets vary over several orders of magnitude as a function of cross-linking.
利用胶体探针原子力显微镜，已在水溶液中测定了玻璃球体与固定化的聚二甲基硅氧烷（PDMS）胶体微滴（具有不同程度的内部交联）之间的排斥力。在初始表面接近时，微滴变形可忽略不计，相互作用力类似于刚性球体的双电层相互作用所预期的力。进一步接近时，微滴变平导致力偏离刚性体双电层相互作用。微滴变形程度已根据与硬球相互作用的偏差来确定。微滴的可变形性强烈依赖于微滴的交联程度，因此由微滴的本体流变学和界面性质的某种组合控制。微滴的纳米流变学已根据力曲线滞后程度来确定。对于低交联水平的类液微滴，未观察到力曲线滞后，弹性变形可用由界面性质控制的单个弹簧常数来描述。对于高度交联的微滴，变形程度由微滴的本体流变学而不是界面性质控制。在表面回缩时，观察到力曲线滞后，这是由于PDMS的粘弹性响应。滞后程度取决于接近/回缩速率和加载力，并已进行理论分析以确定描述微滴松弛过程的纳米流变学参数。PDMS微滴的弹性模量和松弛时间随交联而变化几个数量级。
