Falzone Tobias T, Blair Savanna, Robertson-Anderson Rae M
Department of Physics, University of San Diego, 5998 Alcala Park, San Diego, CA 92110, USA.
Soft Matter. 2015 Jun 14;11(22):4418-23. doi: 10.1039/c5sm00155b.
We drive optically trapped microspheres through entangled F-actin at constant speeds and distances well beyond the linear regime, and measure the microscale force response of the entangled filaments during and following strain. Our results reveal a unique crossover to appreciable nonlinearity at a strain rate of [small gamma, Greek, dot above]c ≈ 3 s(-1) which corresponds remarkably well with the theoretical rate of relaxation of entanglement length deformations 1/τent. Above [small gamma, Greek, dot above]c, we observe stress stiffening which occurs over very short time scales comparable to the predicted timescale over which mesh size deformations relax. Stress softening then takes over, yielding to an effectively viscous regime over a timescale comparable to the entanglement length relaxation time, τent. The viscous regime displays shear thinning but with a less pronounced viscosity scaling with strain rate compared to flexible polymers. The relaxation of induced force on filaments following strain shows that the relative relaxation proceeds more quickly for increasing strain rates; and for rates greater than [small gamma, Greek, dot above]c, the relaxation displays a complex power-law dependence on time. Our collective results reveal that molecular-level nonlinear viscoelasticity is driven by non-classical dynamics of individual entanglement segments that are unique to semiflexible polymers.
我们以恒定速度和远超线性范围的距离驱动光镊捕获的微球穿过缠结的F-肌动蛋白，并测量应变期间及之后缠结细丝的微观尺度力响应。我们的结果揭示了在应变率[小γ，希腊字母，上方有小点]c≈3 s⁻¹时向明显非线性的独特转变，这与缠结长度变形的理论弛豫率1/τent非常吻合。高于[小γ，希腊字母，上方有小点]c时，我们观察到应力强化，其发生在与预测的网眼尺寸变形弛豫时间尺度相当的非常短的时间尺度上。然后应力软化接管，在与缠结长度弛豫时间τent相当的时间尺度上进入有效粘性状态。粘性状态显示出剪切变稀，但与柔性聚合物相比，粘度对应变率的标度不太明显。应变后细丝上感应力的弛豫表明，对于增加的应变率，相对弛豫进行得更快；对于大于[小γ，希腊字母，上方有小点]c的应变率，弛豫显示出对时间的复杂幂律依赖性。我们的总体结果表明，分子水平的非线性粘弹性是由半柔性聚合物特有的单个缠结段的非经典动力学驱动的。
