Near-surface microrheology reveals dynamics and viscoelasticity of soft matter
Near-surface microrheology reveals dynamics and viscoelasticity of soft matter
复制标题
近表面微流变学揭示软物质的动力学和粘弹性
作者：
Liu Wei;Gong Xiangjun;Ngai To;Wu Chi
能够在广泛的时间范围内探测软物质的微观粘弹性特性的实验技术的发展对于揭示控制其行为的物理原理至关重要。在此，我们报告了微流变学技术的发展，该技术可以确定聚合物溶液/凝胶和胶体分散体等软物质的近表面动力学和粘弹性行为。我们的方法将磁场感应刺激器与全内反射显微镜（TIRM）相结合，将机械载荷（∼pN）施加到微米尺寸的探针颗粒上，并以纳米级的灵敏度捕获其表面附近的轴向位移。我们演示了使用该技术来测量胶体与固体基质的分离，并确定胶体-表面分离和相互作用方面不同的三种定量不同的机械耦合机制：排斥、老化和非排斥。我们还通过监测固体表面附近复数模量随频率、时间和间隔距离的演变，将其应用于热敏微凝胶中体积相变的物理凝胶过程和四臂星形聚合物的化学交联溶胶-凝胶转变。与依赖于粒子跟踪的被动微流变技术相比，我们可以探测超过五个数量级的刚度（从 10−3 到 102 Pa）的粘弹性行为，为动力学和异质样品提供出色的覆盖范围。我们预计这项技术将刺激新实验方法的发展，以探索大分子网络、软材料和活细胞质的复杂微观流变学。
The development of experimental techniques able to probe the microscale viscoelastic properties of soft matter over a broad time scale is essential to uncover the physics that govern their behavior. Herein, we report the development of a microrheology technique that can determine the near-surface dynamics and viscoelastic behaviors of soft matter like polymer solution/gels and colloidal dispersions. Our approach combines a magnetic-field-induced stimulator with total internal reflection microscopy (TIRM) to apply mechanical loading (∼pN) to a micro-sized probe particle and capture its axial displacement near the surface with nano-scaled sensitivity. We demonstrate the use of this technique to measure the detachment of a colloid to a solid substrate and identify three quantitatively different regimes of mechanical coupling that differ in colloid–surface separation and interaction: exclusion, aging, and non-exclusion. We also apply it to study a physical gelation process of a volume-phase transition in thermosensitive microgels and a chemically cross-linked sol–gel transition of 4-arm star polymers by monitoring the evolution of complex modulus near solid surface with frequency, time, and separation distance. In contrast to passive microrheology techniques that rely on particle tracking, we can probe the viscoelastic behavior over five orders of magnitude in stiffness, from 10−3 to 102 Pa, providing excellent coverage for dynamics and heterogeneous samples. We expect this technique will stimulate the development of new experimental methods to explore the complex microscale rheology of macromolecular networks, soft materials, and living cytoplasm.