Fluid Dynamics of Nematocysts Firing

Abstract

Extrusomes are specialized extrusive organelles found in cnidarians, protists, and dinoflagellates. They are microscale structures that fire at the largest known acceleration in nature. The fluid dynamics governing these microscale ultra-fast firings are complex due to viscous-dominated effects and boundary layer interactions. Understanding the influence of multiple structures, such as simultaneously or sequentially fired barb(s) or capsule ejection mechanisms, is essential for advancing knowledge of biological propulsion at microscales. In this dissertation, I use the immersed boundary method to numerically simulate the dynamics of both multiple barb firing and capsule ejection in two dimensions. The multiple barb model is of one, two, or three barb-like structures that are centered with a circular prey. The capsule model is modeled as an elliptical shell with a flat plate forming an open ejection end. The enclosed barb is ejected through a contraction or osmotic based ejection mechanism. Reynolds number (Re) is varied across several orders of magnitude to capture a range of viscous to inertia dominated regimes for both models. For multiple-barb firing, I analyze how firing release times and variations in barb spacing influence prey contact. A non-monotonic relationship is shown between the distance of the tip of the center barb to the prey and Re when barbs fire simultaneously. At high Re, inertial forces dominate, preventing the prey from being displaced, thereby ensuring that the center barb consistently reaches its target. In contrast, at lower Re, increased fluid entrainment carries barbs farther, altering their trajectories. I found that prey contact is robust across different firing orders and spacing configurations, emphasizing the dominant role of inertial effects in successful contact at high Re. For the capsule ejection model, I analyze key parameters influencing ejection efficiency, including the minor axis length of the elliptical shell, the gap opening size, and the material properties of the barb. Results show that decreasing the gap size generates higher pressure gradients, leading to faster ejection times. Similarly, increasing the minor axis of the capsule facilitates more rapid target contact by expelling a greater volume of fluid. At shorter distances, the relationship between time to contact and Re is non-monotonic, whereas at larger initial separations, as Re increases the time to reach the prey decreases. Higher Re values also enhance the robustness of prey contact across various capsule configurations. Additionally, I analyze how barb stiffness affects trajectory and impact time. The behavior demonstrated that flexible barbs undergo significant deformation, and stiffer barbs reach the target more efficiently. Overall, these models reveal the extreme non-linearity of fluid dynamics at this scale, leading to complex, non-monotonic relationships between prey contact and system parameters. However, high Re enables robustness in barb firing and capsule ejection, ensuring eventual contact with the target. These findings provide fundamental insights into the biomechanics of extrusome function and contribute to broader applications in microinjector technology, where precise and rapid ejection is necessary.