光镊微观流变学绘制缠结聚合物中应变引起的局部不均匀性的动力学
Optical tweezers microrheology maps the dynamics of strain-induced local inhomogeneities in entangled polymers
摘要：
光镊微流变学（OTM）提供了一种强有力的方法来探测复杂的软物质系统，如网络的纠缠聚合物，在广泛的时空尺度的非线性响应。OTM还可以独特地表征导致这些系统表现出的有趣的非线性流变特性的微观结构动力学。然而，OTM测量中的应变，通过光学迫使微探针通过材料，诱导网络的不均匀性和周围的应变路径，和所得的流场复杂的系统的测量响应。通过一组强大的定制设计的OTM协议，再加上建模和分析计算，我们表征的时变不均匀性引起的OTM测量领域。我们表明，应变后均匀化不干扰系统的内在应力松弛动力学，而是它表现为一个独立的组成部分，在应力衰减，即使在高度非线性制度，如微流变LAOS（mLAOS）协议，我们介绍。我们的具体结果表明，类罗斯弹性回缩，而不是解缠结和脱离，主导缠结聚合物在微观和介观尺度的非线性应力松弛。因此，我们的研究开辟了在广泛的复杂大分子系统上进行精确的非线性微流变测量的可能性，例如mLAOS。
英文摘要：
Optical tweezers microrheology (OTM) offers a powerful approach to probe the nonlinear response of complex soft matter systems, such as networks of entangled polymers, over wide-ranging spatiotemporal scales. OTM can also uniquely characterize the microstructural dynamics that lead to the intriguing nonlinear rheological properties that these systems exhibit. However, the strain in OTM measurements, applied by optically forcing a micro-probe through the material, induces network inhomogeneities in and around the strain path, and the resultant flow field complicates the measured response of the system. Through a robust set of custom-designed OTM protocols, coupled with modeling and analytical calculations, we characterize the time-varying inhomogeneity fields induced by OTM measurements. We show that post-strain homogenization does not interfere with the intrinsic stress relaxation dynamics of the system, rather it manifests as an independent component in the stress decay, even in highly nonlinear regimes such as with the microrheological-LAOS (mLAOS) protocols we introduce. Our specific results show that Rouse-like elastic retraction, rather than disentanglement and disengagement, dominates the nonlinear stress relaxation of entangled polymers at micro- and meso- scales. Thus, our study opens up possibilities of performing precision nonlinear microrheological measurements, such as mLAOS, on a wide range of complex macromolecular systems.
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