Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session D39: Bio: Plant Mechanics |
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Chair: Sunghwan Jung, Virginia Tech University Room: Portland Ballroom 256 |
Sunday, November 20, 2016 2:57PM - 3:10PM |
D39.00001: Active flows on trees Aden Forrow, Francis G. Woodhouse, Jörn Dunkel Coherent, large scale dynamics in many nonequilibrium physical, biological, or information transport networks are driven by small-scale local energy input. We introduce and explore a generic model for compressible active flows on tree networks. In contrast to thermally-driven systems, active friction selects discrete states with only a small number of oscillation modes activated at distinct fixed amplitudes. This state selection can interact with graph topology to produce different localized dynamical time scales in separate regions of large networks. Using perturbation theory, we systematically predict the stationary states of noisy networks. Our analytical predictions agree well with a Bayesian state estimation based on a hidden Markov model applied to simulated time series data on binary trees. While the number of stable states per tree scales exponentially with the number of edges, the mean number of activated modes in each state averages $\sim 1/4$ the number of edges. More broadly, these results suggest that the macroscopic response of active networks, from actin-myosin networks in cells to flow networks in Physarum polycephalum, can be dominated by a few select modes. [Preview Abstract] |
Sunday, November 20, 2016 3:10PM - 3:23PM |
D39.00002: Low Reynolds number flow near tiny leaves, stems, and trichomes Christopher Strickland, Virginia Pasour, Laura Miller In terrestrial and aquatic environments such as forest canopies, grass fields, and seagrass beds, the density and shape of trunks, branches, stems, leaves and trichomes (the hairs or fine outgrowths on plants) can drastically alter both the average wind speed and profile through these environments and near each plant. While many studies of flow in these environments have focused on bulk properties of the flow at scales on the order of meters, the low Reynolds number flow close to vegetative structures is especially complex and relevant to nutrient exchange. Using three-dimensional immersed boundary simulations, we resolve the flow around~trichomes and small~leaves and quantify velocities, shear stresses, and mixing while varying the height and density of idealized structures. [Preview Abstract] |
Sunday, November 20, 2016 3:23PM - 3:36PM |
D39.00003: Effect of wind-induced drag on leaf shapes Jean-Francois Louf, Pierre Ntoh Song, Tim Zehnbauer, Sunghwan Jung Under windy conditions everyone can see leaves bending and twisting. From a geometrical point of view, a leaf is composed of two parts: a large flat plate called the lamina, and a small beam called the petiole, connecting the lamina to the branch/stem. While the wind is exerting forces (e.g. drag) on the lamina, the petiole undergoes twisting and bending stresses. To survive in harsh abiotic conditions, leaves might have evolved to form in many different shapes, resulting from a coupling between the lamina and the petiole. In this study we measure the twisting modulus (G) of the petiole using a twisting setup, and its Young modulus (E) by performing tensile tests. Micro-CT scan is used to precisely measure the cross section of the petiole allowing us to calculate the second moment of inertia (I) and the second moment of area (J). We then use the non-dimensional number EI/GJ and compare it to a geometrical non-dimensional number (Lpetiole$+$Llamina/2)/W, where Lpetiole is the length of the petiole, Llamina the length of the lamina, and W the width of the lamina. We found a linear relation between the ratio of the bending to twisting rigidity and the leaf geometry. [Preview Abstract] |
Sunday, November 20, 2016 3:36PM - 3:49PM |
D39.00004: Smart change in leaf morphology to tune the wettability Hosung Kang, Sara Fleetwood, Sunghwan Jung Plants are sessile organisms, but some of them are able to change their features to survive. We found Cercidiphyllum japonicum (Katsura) leaves actively adapt to their fine structures on the leaf surface in response to external stimuli. It is fascinating how the structural changes can affect their physical properties. In this present study, we are investigating the effect of external environments (temperature, cell hydration, and acid rain) on microscale papillose epidermal cells and nanoscale waxes. Using environmental scanning electron microscopy (ESEM) and atomic force microscopy (AFM), we measured micro and nano structures of the Katsura leaves. We found a functional relation between the micro and nano structures and the contact angle of the leaf's surface. As the epidermal cells shrink and the waxes erode, the contact angle decreases. A simple Cassie-Baxter model based on the wettability of textured surfaces has been used to characterize changes of the contact angle. [Preview Abstract] |
Sunday, November 20, 2016 3:49PM - 4:02PM |
D39.00005: Stomatal design principles for gas exchange in synthetic and real leaves Kaare H. Jensen, Katrine Haaning, C. Kevin Boyce, Maciej Zwieniecki Stomata are portals in plant leaves that control gas exchange for photosynthesis, a process fundamental to life on Earth. Gas fluxes and plant productivity depend on external factors such as light, water, and CO$_2$ availability and on geometric properties of the stomata pores. The link between stomata geometry and environmental factors have informed a wide range of scientific fields -- from agriculture to climate science, where observed variations in stomata size and density is used to infer prehistoric atmospheric CO$_2$ content. However, the physical mechanisms and design principles responsible for major trends in stomatal patterning, are not well understood. Here we use a combination of biomimetic experiments and theory to rationalize the observed changes in stomatal geometry. We show that the observed correlations between stomatal size and density are consistent with the hypothesis that plants favor efficient use of space and maximum control of dynamic gas conductivity, and -- surprisingly -- that the capacity for gas exchange in plants has remained constant over at least the last 325 million years. Our analysis provides a new measure to gauge the relative performance of species based on their stomatal characteristics. [Preview Abstract] |
Sunday, November 20, 2016 4:02PM - 4:15PM |
D39.00006: Dynamics of a freely-falling maple seed Injae Lee, Haecheon Choi We conduct numerical simulations of a freely-falling maple seed using an immersed boundary method in a non-inertial reference frame (Kim and Choi, JCP, 2006). A three-dimensional seed model is obtained by scanning a maple seed. The seed reaches a steady autorotation after a transient period, and a stable leading-edge vortex is attached on the surface of the rotating seed, which increases the drag force during autorotation. In addition, two different approaches are considered to obtain scaling laws describing the relation among the seed weight and geometry, and descending and rotating velocities. The first uses the conservations of mass, linear and angular momentum, and energy. In this approach, a model constant to be determined, called axial induction factor, is obtained from the result of present simulation. The second approach employs a classical steady wing theory in which the vortical strength is scaled with the circulation around a wing and the lift force is modeled by the time derivative of vortical impulse (Lee \textit{et al}., JFM, 2015). Available data on various seeds well fall on these scaling laws. [Preview Abstract] |
Sunday, November 20, 2016 4:15PM - 4:28PM |
D39.00007: The flight of {\em Ruellia ciliatiflora} seeds Dwight Whitaker, Eric Cooper, Molly Mosher, Yijun Wang, Chaelee Dalton The fruits of {\em Ruellia ciliatiflora} open explosively and launch mm-sized disks at speeds exceeding 10 m/s a distance of 5 m. Observations with high-speed video reveal that the seeds are launched in a streamline orientation that is maintained with a backspin of 1.5 kHz. Through a careful analysis of the high-speed videos of the seeds' flight we measure the aerodynamic forces on these spinning seeds. We find that the exceptional rotation rate both reduces drag on the seed by keeping its cross section as small as possible and generates a modest ($\sim$ 0.3$g$) lift on the flying seeds. To understand the aerodynamic forces we create photometrically scanned, 3D printed models of the seeds for particle image velocimetry (PIV) in a flume of tow tank. We will discuss our method for producing accurately shaped model seeds as well as preliminary PIV data on the flow of fluid around the flying seed. This work marks the start of a longer-term project that will compare the dynamics of seed launch and flight within the Acanthaceae family, which has over 2000 species in habitats ranging from rainforest to savannah that all use a similar method for launching seeds. [Preview Abstract] |
Sunday, November 20, 2016 4:28PM - 4:41PM |
D39.00008: Fluid mechanics of osmotic pipe flows and limitations on the lengths of conifer needles. Tomas Bohr, Hanna Rademaker, Kaare Jensen, Maciej Zwieniecki Plant leaves produce sugars, which are exported osmotically through the sieve tubes of the leaf. Leaf sizes vary by more than 3 orders of magnitude, from a few millimeters to over one meter. Conifer leaves (needles), however, are relatively short and the majority of needles are no longer than 6 cm. The reason for this limitation is unknown, but we argue that it can be explained by the linear venation pattern and the narrow sieve tubes, combined with the osmotic flow mechanism. Thus sugars produced near the tip of long needles cannot be exported efficiently, because the pressure required to drive vascular flow would exceed the greatest available pressure (the osmotic pressure). This basic constraint leads to the formation of an inactive region of stagnant fluid near the needle tip, which does not contribute to sugar flow. The active region, emerging from the base of the needle, has the length $L_{eff} =r^{3/2}\left( {16\eta L_{p} } \right)^{1/2}$, where $r$ is the conduit radius, $\eta $ is the sap viscosity, and $L_{p}$ is the cell membrane permeability. It is independent of the needle length and corresponds well with maximal needle lengths observed in nature. [Preview Abstract] |
Sunday, November 20, 2016 4:41PM - 4:54PM |
D39.00009: The life of bubbles under negative pressure Jin Woo Choi, Keunhwan Park, So Nagashima, Myoung-Woon Moon, Ho-Young Kim Cavitation of sap in plant vessels, or embolism, may occur when the liquid pressure becomes negative either in a high elevation or a dry environment. Effective suppression of nucleation and growth of cavitation bubbles is important for continuous transport of water and thus survival of the plant. Here we investigate the life of cavitation bubbles under negative pressure from their nucleation through growth and maturation. As a model system for the plant vessel, we fabricate hydrogel microchannels whose inner pressure is reduced to a negative value. The roughness of the channel surface is modified by plasma treatment to form wrinkles emulating observed xylem wall surfaces. We find a finite effect of surface wrinkles on the critical nucleation pressure. Also, dense wrinkles tend to slow down bubble growth. In all the channel roughness conditions, the bubbles grow diffusively with time until their maturation. Then in the matured stage, the growth speed is substantially lowered and follows the value determined by Darcy’s law. Our results suggest that surface wrinkles or roughness can be used to control the nucleation pressure and bubble growth behavior. Also, the observations can give deeper insight into embolism control mechanisms of tall trees. [Preview Abstract] |
Sunday, November 20, 2016 4:54PM - 5:07PM |
D39.00010: Breakdown of water transport and resilient xylem structure in vascular plants Jeongeun Ryu, Wonjung Kim, Sang Joon Lee Plants can transport sap water without using a mechanical pump by exploiting a metastable state of water. However, sap water in a metastable state is vulnerable to cavitation and embolism, disrupting water transport in xylem vessels. We note that under this paradox, plants have been evolved to have resilient xylem network against breakdown of water transport as a survival strategy. In this study, we directly observe the onset of embolism and its spreading dynamics in live plants to establish a synthetic tree model. We also rationalize our experimental findings with a model describing embolism propagation under a metastable state of water and an interconnected xylem network structure which can minimize damages from cavitation and embolism. This study would shed light on the design of complex networks with resilience for effective transport as well as the physical understanding on the transport of metastable water. [Preview Abstract] |
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