Department of Mechanical Engineering, Santa Clara University, Santa Clara, California 95053, USA.
Phys Rev E. 2017 Dec;96(6-1):062606. doi: 10.1103/PhysRevE.96.062606. Epub 2017 Dec 11.
Micro-organisms expend energy moving through complex media. While propulsion speed is an important property of locomotion, efficiency is another factor that may determine the swimming gait adopted by a micro-organism in order to locomote in an energetically favorable manner. The efficiency of swimming in a Newtonian fluid is well characterized for different biological and artificial swimmers. However, these swimmers often encounter biological fluids displaying shear-thinning viscosities. Little is known about how this nonlinear rheology influences the efficiency of locomotion. Does the shear-thinning rheology render swimming more efficient or less? How does the swimming efficiency depend on the propulsion mechanism of a swimmer and rheological properties of the surrounding shear-thinning fluid? In this work, we address these fundamental questions on the efficiency of locomotion in a shear-thinning fluid by considering the squirmer model as a general locomotion model to represent different types of swimmers. Our analysis reveals how the choice of surface velocity distribution on a squirmer may reduce or enhance the swimming efficiency. We determine optimal shear rates at which the swimming efficiency can be substantially enhanced compared with the Newtonian case. The nontrivial variations of swimming efficiency prompt questions on how micro-organisms may tune their swimming gaits to exploit the shear-thinning rheology. The findings also provide insights into how artificial swimmers should be designed to move through complex media efficiently.
微生物在复杂介质中移动时会消耗能量。虽然推进速度是运动的一个重要特性，但效率是另一个因素，它可能决定微生物为了以节能的方式运动而采用的游动步态。不同的生物和人工游泳者在牛顿流体中游泳的效率已经得到很好的描述。然而，这些游泳者经常遇到表现出剪切稀化粘度的生物流体。对于这种非线性流变学如何影响游动效率，人们知之甚少。剪切稀化流变学使游泳更高效还是低效？游泳效率如何取决于游泳者的推进机制和周围剪切稀化流体的流变学特性？在这项工作中，我们通过考虑 squirmer 模型作为一种通用的运动模型来代表不同类型的游泳者，来解决剪切稀化流体中运动效率的这些基本问题。我们的分析揭示了在 squirmer 上选择表面速度分布如何可以减少或提高游泳效率。我们确定了可以与牛顿情况相比大幅提高游泳效率的最佳剪切率。游泳效率的重要变化引发了关于微生物如何调整游动步态以利用剪切稀化流变学的问题。这些发现还为如何设计人工游泳者以有效地通过复杂介质提供了思路。
