Session H11: Biological Fluid Dynamics: Plant Biomechanics

Fungi can measure wind speeds

The filamentous fungus Neurospora produces spores profusely on specialized stalk cells called aerial hyphae. The length of these cells is affected by a physical tradeoff: longer cells may see faster air speeds, but are also more likely to buckle under their own weight. In particular, best dispersal requires that aerial hyphae extend through the boundary layer of still air that clings to the parent fungus to reach the moving air flows beyond. Boundary layer thickness is controlled by wind speed, but how can wind speed be measured by the fungus’s microscopic cells? Our experiments growing fungi in wind tunnels show that aerial hyphae length varies to match boundary layer thicknesses. Further, we find that the cells’ ability to respond to changes in evaporation rates in the key to their microbial anemometry.    

Fluttering leaf to quantify internal water stress

Water stress in plants is a predominant factor that directly affects essentials such as growth, photosynthesis, and crop yield. It is always crucial to evaluate the water stress in plants and supply adequate treatments, including cases of food problems and drought. However, many traditional methods such as using a sensor insertion or soil moisture test have a chance to be inaccurate or injure the plant. Here, we propose a noninvasive way to measure the water stress of plants by observing the bending and twisting motion of leaves. Experimentally, we observe the frequency and amplitude of leaf movement by dropping small balls on the tip and sides of the lamina and adapt beam theory to predict the movement. As a result, our theoretical model matches with experimental measurements regarding the frequency and maximum amplitude of both bending and twisting. Moreover, a strong correlation between the water stress and the natural frequency of the leaf is characterized. This result implies a connection of the stiffness of leaves according to water stress, so it could be utilized as an overall indicator of plant performance.   

Lagrangian coherent structures in spore dispersal around fluttering leaves

Plant pathogens such as rust spores cause ongoing issues for agricultural practices. Past studies have shown that liberation of rust spores from leaf surfaces can result from vortices induced by impacting droplets. New experimental findings on oscillatory substrates like a fluttering leaf suggest that periodic shedding of counter-rotating vortex tubes can enhance particle mixing and spatial transport. The navigation routes of shed particles from sheared boundary layers can be described by the centrifugation/expulsion from Lagrangian coherent vortices and the further expulsion of the outer conveyer belts formed by deforming vortex dipoles. We used both Lagrangian-averaged vorticity deviation (LAVD) calculations and Finite Time Lyapunov Exponents (FTLE) ridges to identify such elliptical and hyperbolic Lagrangian coherent structures (LCS) respectively, which emerge over 2D flow maps on the transverse cross-section of the leaf vibration, while investigating particle advection properties owing to such oscillatory, unsteady flow patterns. The vorticities are extracted from smoke visualization data and matched with theoretical predictions based on 2D velocity potentials commonly adopted in oscillatory airfoil theory. Dispersal statistics were collected for the dynamical process. In summary, the study visually captures vortical airflow patterns hidden in natural leaf vibrations, applies dynamics concepts to study its dry advection properties, and compares experimental data on transport with theoretical estimates.   

Not Participating

Leaves may flutter in light winds for the purpose of convective and evaporative heat transfer. We will simulate the passive fluid-structure interaction of leaves fluttering using the immersed boundary method. Computational simulations have been conducted in two dimensions using the software library IB2d. We will discuss the models developed to appropriately study the heat dissipation and how we have incorporated these thermal dynamics coupled with the fluid flow into IB2d. We will present results of the simulations where we have studied the impact of varying Reynolds and Peclet numbers.   

Raindrop impact location predicts seed dispersal distances in rain-activated splash cup plants

Splash cup plants exhibit rain-activated seed dispersal in which raindrops impact millimeter-sized cup-shaped fruiting bodies. The resulting splash can carry seeds up to a meter away from the plant. Previous research has examined this dispersal mechanism in vivo as well as with 3D-printed model cups. Previous work determined the optimal cone shape for the cups and showed that raindrop impacts that hit off-center led to longer dispersal distances than on-center. However, the effect of impact location was not systematically studied. Here, we build upon this work by systematically studying how raindrop impact location affects seed dispersal distances. We performed experiments across a range of 3D-printed biomimetic conical cup shapes and hydrophobic glass beads that mimic seeds. Our results show that the impact location of the incoming raindrop is a significant predictor of seed dispersal distances. We find that seed dispersal distance varies with impact location, cone shape, and the number of seeds in the cup. These results suggest that raindrop impact location must be accounted for when measuring dispersal, and that even small (< 0.5mm) departures in impact location have significant effects on the dispersal distances of seeds.   

The speed of osmotically driven laminar flow within elastic semi-permeable tubes

The phloem provides the pathway for the products of photosynthesis to be transported to different parts of the plant for consumption or storage. The Munch pressure flow theory is currently the most accepted to mathematically represent sucrose transport. It is based on osmosis that provides the pressure gradient to drive the flow within the phloem. Due to experimental challenges, mathematical models are used to complement and assist interpreting plant responses to environmental stresses that reduce the efficiency of phloem transport. One of these simplifications is to geometrically represent the phloem as a rigid semi-permeable tube. However, the phloem consists of living cells that contract and expand in response to pressure fluctuations. The effect of membrane elasticity on sucrose front speed frames the scope of the talk. Laboratory experiments are first conducted to elucidate the pressure-radius relation in membranes. The findings are then incorporated into 2-D numerical simulations. It is demonstrated that the speed of the flow is decreased when the tube is allowed to expand. These results offer a novel perspective about the role of sieve plates evenly spaced throughout the phloem.   

Turbulence-induced Reconfiguration of Fractal-Branched Trees in a Forest

The reconfiguration of trees and their ability to redistribute and damp aerodynamic forces are essential for their continual survival. In addition, the collective dynamics of trees and their interaction with the atmosphere flow could cause large-scale flow instability and the emergence of coherent wind gusts. In this work, we study the coupling of turbulent flow with rigid and flexible fractal trees in a forest to predict the role of tree flexibility in changing flow field and the creation of large coherent structures. A branched tree is modelled as a fractal multilink dynamic system and large eddy simulation is performed to investigate the interaction of many trees in a canopy with the atmospheric boundary layer. We will show the feedback mechanism between the formation of coherent vortex structures on top of the canopy, substantial dynamic opening in the canopy of reconfigured trees, and vertical drift of coherent flow structures due to more intense fluid-structure interactions. Finally, we discuss how the feedback response between the canopy and boundary layer is affected by branching pattern of trees and their deformability.   

Fluid-structure interactions in soft channel networks

Fluid-structure interactions in soft channels play a critical role in fluid flow control in plants and in the human brain and kidneys. While the properties of individual flexible channels are well-known, flow in interconnected or mechanically coupled pipe networks remain poorly understood.

To elucidate the basic modes of self-interacting channels, we study pressure-driven flow in a multi-layer fluidic device that contains two overlapping channels separated by an impermeable elastic membrane. 

This allows us to build networks of capacitors and non-linear resistors as models of biological systems. Implications for flow control in plant xylem is discussed and potential technical applications are outlined.    

Diffusion and flow across shape-perturbed plasmodesmata nanopores in plants

Plasmodesmata are slender nanochannels that link neighboring plant cells and enable the exchange of nutrients and signaling molecules. Recent experiments have demonstrated significant variability in the concentric pore shape. However, the impact of these geometric fluctuations on transport capacity is unknown. Here, we consider the effects on diffusion and advection of two ideal shape-perturbations: a radial displacement of the entire central desmotubule and a harmonic variation in the cytoplasmic sleeve width along the length of the pore. We use Fick's law and the lubrication approximation to determine the diffusive current and volumetric flow rate across the pore. Our results indicate that an off-center desmotubule always increases the pressure-driven flow rate. However, the diffusive current is only enhanced for particles comparable in size to the width of the channel. In contrast, harmonic variations in the cytoplasmic sleeve width along the length of the pore reduce both the diffusive current and the pressure-driven flow. The simple models presented here demonstrate that shape-perturbations can significantly influence transport across plasmodesmata nanopores.    

A multi-stimuli responsive, growing liquid robot

Stimuli responsive soft robots are recently gaining significant interest in a range of applications including flexible electronics and biomedicine. However, most soft robots are still limited in their responses to responding to stimuli by deforming in a pre-programmed, nastic manner. Therefore, implementing the adaptability of living organisms, which grow and actively respond to the environmental stimuli, in an artificial system is an important scientific challenge that remains to be solved. Inspired by tip growing cells of nature, here we report a multi-stimuli responsive, growing liquid robot based on non-solvent induced phase separation of polymeric solution. Because the growth is driven by the balance between internal pressure and wall strength as in pollen tubes and fungal hyphae, the size of the robot can be precisely controlled during growth. The robot can sense the direction of multiple stimuli–gravity, mechanical contact, and light–and adjust its growing direction. Owing to its unique characteristics, the robot can transport another liquid, print 3D structures in a confined space, and bypass mechanical obstacles.