Swimming behaviour of a flagellated alga in Newtonian and complex fluids

  • Ewan Rycroft

Student thesis: Doctoral Thesis

Abstract

Many microscopic swimmers in nature navigate through complex fluids, such as worm typeswimmers in muds, and spermatozoa in cervical mucus. An understanding of the swimmingresponse to such fluids is gaining increasing attention with the hope they will aid inthe development of artificial microswimmers and in enhancing processes involving naturalbioswimmers, such as fertility treatments. In this work, the swimming behaviour of aswimming green algae, Dunaliella salina, is examined experimentally. Fluids with differentrheological properties have been used to study the e ects of increasing viscosity, shearthinning properties and viscoelasticity of the surrounding medium on the algae swimmingcharacteristics, such as velocity, beating frequencies and stroke velocities.In a water-like medium akin to their natural environment, the algae were found to swimwith a velocity of Vnet = 49:55 µms⁻¹ while beating at a frequency of fBF = 29:97 Hz. Withthe addition of a viscous enhancing agent (Ficoll PM400) the algae swimming velocityfollowed an essentially monotonic decrease as viscosity increased, based on a power-lawrelationship (V x ƞ⁻¹). This was attributed to the constant drag produced by the algaeand limited variations in the stroke dynamics, which was confirmed by analysis of thepower and recovery strokes.Compared to the Newtonian cases, when the surrounding fluid exhibited shear thinningproperties, achieved by addition of Xanthan gum, the swimmer displayed reduced stroketimes for comparable displacements in the Newtonian cases. This lead to an overall boostin the recovery stroke over the power stroke. Polyacrylamide was used to analyse theviscoelastic response of the algae. However, for this particular case it was apparent thatdue to the confounding effects of elasticity and shear thinning, it was difficult to define an elastic response.Furthermore, the wall interactions of Dunaliella salina was quantified, with a preferenceto bounce from a wall observed. The approach dynamics to the wall were found to holdlittle influence on the escape, with only a slight tendency for increased reverse bounces athigh approach angles and slower velocities.
Date of Award23 Feb 2022
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsEPSRC (Engineering and Physical Sciences Research Council)
SupervisorMonica Oliveira (Supervisor) & Mark Haw (Supervisor)

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