Avital Yotam Y, Farago Oded
Department of Biomedical Engineering and Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev, Be'er Sheva 84105, Israel.
J Chem Phys. 2015 Mar 28;142(12):124902. doi: 10.1063/1.4915512.
We use computer simulations and a simple free energy model to study the response of a bilayer membrane to the application of a negative (compressive) mechanical tension. Such a tension destabilizes the long wavelength undulation modes of giant vesicles, but it can be sustained when small membranes and vesicles are considered. Our negative tension simulation results reveal two regimes-(i) a weak negative tension regime characterized by stretching-dominated elasticity and (ii) a strong negative tension regime featuring bending-dominated elastic behavior. This resembles the findings of the classic Evans and Rawicz micropipette aspiration experiment in giant unilamellar vesicles (GUVs) [E. Evans and W. Rawicz, Phys, Rev. Lett. 64, 2094 (1990)]. However, in GUVs the crossover between the two elasticity regimes occurs at a small positive surface tension, while in smaller membranes it takes place at a moderate negative tension. Another interesting observation concerning the response of a small membrane to negative surface tension is related to the relationship between the mechanical and fluctuation tensions, which are equal to each other for non-negative values. When the tension decreases to negative values, the fluctuation tension γ drops somewhat faster than the mechanical tension τ in the small negative tension regime, before it saturates (and becomes larger than τ) for large negative tensions. The bending modulus exhibits an "opposite" trend. It remains almost unchanged in the stretching-dominated elastic regime, and decreases in the bending-dominated regime. Both the amplitudes of the thermal height undulations and the projected area variations diverge at the onset of mechanical instability.
我们使用计算机模拟和一个简单的自由能模型来研究双层膜在施加负（压缩）机械张力时的响应。这种张力会破坏巨型囊泡的长波长波动模式，但在考虑小膜和囊泡时它可以持续存在。我们的负张力模拟结果揭示了两种状态——（i）以拉伸主导弹性为特征的弱负张力状态，以及（ii）以弯曲主导弹性行为为特征的强负张力状态。这类似于经典的埃文斯和拉维茨在巨型单层囊泡（GUVs）中进行的微吸管抽吸实验的结果[E. 埃文斯和W. 拉维茨，《物理评论快报》64, 2094 (1990)]。然而，在GUVs中，两种弹性状态之间的转变发生在一个小的正表面张力处，而在较小的膜中，它发生在一个适度的负张力处。另一个关于小膜对负表面张力响应的有趣观察结果与机械张力和涨落张力之间的关系有关，对于非负值，它们彼此相等。当张力减小到负值时，在小负张力状态下，涨落张力γ下降得比机械张力τ稍快，然后在大负张力时达到饱和（并变得大于τ）。弯曲模量呈现出“相反”的趋势。它在拉伸主导的弹性状态下几乎保持不变，而在弯曲主导的状态下减小。在机械不稳定开始时，热高度起伏的幅度和投影面积变化都会发散。
