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In designing structural elements and machine parts using ductile materials, it is crucial to ensure that these components withstand applied stresses without yielding. Yielding is initially determined through a tensile test, which evaluates the material's response to uniaxial stress. However, tensile stress is insufficient when components face biaxial or plane stress conditions This condition requires advanced criteria to predict failure.
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The stress-strain relationship in ductile materials such as structural steel or aluminium is intricate and progresses through several stages. When a specimen is loaded, it initially exhibits a linear length increase, depicted by a steep straight line on the stress-strain diagram. It indicates the material is elastically deforming and will return to its original shape once unloaded. However, when a critical stress value is reached, plastic deformation begins. This stage sees substantial...
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Helical Locomotion in Yield Stress Fluids.

Farshad Nazari1, Kourosh Shoele2, Hadi Mohammadigoushki1

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Locomotion in yield stress fluids requires overcoming yield strain for rotation and sufficient viscous forces for plastic deformation. Yield stress impacts helical swimmer speed, enhancing it at large pitch angles but retarding it at small ones.

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Area of Science:

  • Fluid dynamics
  • Rheology
  • Soft matter physics

Background:

  • Understanding microswimmer dynamics is crucial for applications in targeted drug delivery and micro-robotics.
  • Yield stress fluids exhibit a critical stress threshold before flowing, complicating traditional fluid dynamics models.
  • Helical swimmers are a common model for self-propelled micro-objects.

Purpose of the Study:

  • To investigate the distinct stages and mechanisms governing the locomotion of a helical swimmer in yield stress fluids.
  • To elucidate the role of yield stress and viscous forces in enabling and modulating swimmer motion.
  • To characterize the flow field generated by the swimmer in these non-Newtonian fluids.

Main Methods:

  • Experimental observation of a helical swimmer's motion.
  • Rheological characterization of the fluid medium.
  • Flow visualization techniques to analyze fluid deformation around the swimmer.

Main Results:

  • Three distinct stages of locomotion were identified: yield strain overcoming, plastic deformation for net motion, and modulated swimming.
  • Locomotion initiation requires exceeding the yield strain and sufficient viscous forces for plastic deformation.
  • Yield stress significantly influences swimming speed, retarding motion at small pitch angles and enhancing it at large pitch angles.
  • Flow visualization revealed highly localized flow patterns near the swimmer.

Conclusions:

  • The study defines critical thresholds and mechanisms for helical swimmer locomotion in yield stress fluids.
  • The findings highlight the complex, non-monotonic relationship between yield stress and swimming performance based on pitch angle.
  • Localized flow fields are characteristic of swimmer-fluid interaction in yield stress materials.