| Author: | Liu, Jinan |
| Title: | Propulsion and steering of artificial flagellated micro-swimmers |
| Advisors: | Ruan, Haihui (ME) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Microrobots Microorganisms Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Mechanical Engineering |
| Pages: | xxviii, 173 pages : color illustrations |
| Language: | English |
| Abstract: | This thesis achieves a quantitative model of artificial flagellated micro-swimmers (AFMSs) to predict the motility and steerability of an acoustically actuated AFMS. We first summarized the historical achievements and theoretical perspectives on microorganisms and their propulsion mechanisms, then reviewed the progress in actuation strategies of AFMSs, and finally focused on a simple sperm-like micro-swimmer geometry, composed of an ellipsoidal head and a flagellum (tail) with the length of hundreds of micrometers. We argue that these AFMSs can swim by inducing head oscillations that beat the flagellum to achieve wavy motion and thus the propulsion in a low Reynolds number (LRN) Newtonian fluid environment. We provided the quantitative relation between head oscillation amplitude and acoustic pressure and frequency, and the theoretical account of how the flagellum is whipped, bringing about propulsion. The one-dimensional (1D) equations of motion (EOM) for a flagellum, treated as an Euler-Bernoulli viscoelastic beam, were then derived based on the resistive force theory (RFT) and solved by using the Galerkin method. In order to make our theoretical model applicable for designing the AFMS, we have involved the inertia term and material damping in the 1D EOM and considered the tapered cross-section of a flagellum. The numerical results reveal that the micro-swimmer actuated by ultrasound can achieve a perceptible velocity, especially at resonance. Influences of nondimensional parameters, such as the resonance index, sperm number, and material damping coefficient, were discussed and a comparison with reported experimental results demonstrates the validity of the proposed 1D model. To deal with the steerability of micro-swimmers under magneto-acoustic actuation with significant non-linearity in EOM, we proposed a bar-joint model based on the corrected resistive force theory (CRFT) for studying AFMSs propelled in a 2D acoustic field or with a rectangular cross-section. Note that the classical RFT for 3D cylindrical flagellum leads to over 90% deviation in terminal velocities from those of 2D fluid-structure interaction (FSI) simulations, while the proposed CRFT bar-joint model can reduce the deviation to below 5%; hence, it enables a reliable prediction of the 2D locomotion of an acoustically actuated AFMS with a rectangular cross-section, which is the case in many experiments. Introduced in the CRFT is a single correction factor K determined by comparing the linear terminal velocities under acoustic actuation obtained from the CRFT with those from simulations. After the determination of K, detailed comparisons of trajectories between the CRFT-based bar-joint AFMS model and the FSI simulation were presented, exhibiting excellent consistency. Finally, a numerical demonstration of the purely acoustic or magneto-acoustic steering of an AFMS based on the CRFT was presented, which can be one of the choices for future AFMS-based precision therapy. Experimentally, AFMSs can be manufactured based on digital light processing (DLP) of UV-curable resins. We first determined the viscoelastic properties of a UV-cured resin through dynamic mechanical analysis (DMA). The high-frequency storage moduli and loss factors were obtained based on the assumption of time-temperature superposition (TTS), which were then applied in theoretical calculations. Though the extrapolation based on the TTS implied the uncertainty of high-frequency material response and there is limited accuracy in determining head oscillation amplitude, the differences between the measured terminal velocities of the AFMSs and the predicted ones are less than 50%, which, to us, is well acceptable. These results indicate that the motions of acoustic AFMS can be predicted, and thus, designed, which pave the way for their long-awaited applications in targeted therapy. |
| Rights: | All rights reserved |
| Access: | open access |
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