“Collector motion affects particle capture in physical models and in wind pollination”

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Dori McCombe and Josef D. Ackerman (July 2018)

The DOI will be https://dx.doi.org/10.1086/697551

Particle capture efficiency on moving collectors determined by flow (Reynolds no.) and particle dynamics (Stokes no.)

A moving flower captures more pollen – why collector motion is important to particle capture

Above, bright field image of pollen capture on Phleum pratense stigmas from transverse movement treatment on June 28, 2014. Below, epifluorescent image of pollen capture on Phleum pratense stigmas from transverse movement treatment on June 28, 2014. Note the pollen tube germination and growth.
(Credit: Dori McCombe)

Many ecological processes, such as wind and water pollination and filter feeding, involve the capture of particles suspended in a fluid – air and water. Despite the ecological and economic importance of these processes (e.g., many of our crop plants and trees are pollinated by wind), particle capture has been mostly studied on stationary rather than moving collectors. This is somewhat surprising given the flow-induced movements of plants visible on windy days.

Using some insights from numerical models of particle capture on moving collectors, researchers at the University of Guelph have expanded this field by using a combination of empirical studies of a physical model in the lab and a wind pollination study in a field of timothy grass (Phleum pretense) in which they experimentally restricted the motion of plants in different ways.

They found that moving collectors captured more particles than stationary ones in the lab as did moving plants in the field. The scale of the impact, however, depended on the flow environment around the moving collector, and the characteristics of the particle. This realization was used to resolve a controversy about the importance of plant movement in pollination success and the ways in which pollen are captured in the wind.

Incorporating the ecology of flow-induced movements into the study of particle capture provides a better understanding of the rate and importance of these biological processes, which have been hitherto underestimated.


Abstract

Particle capture is important for ecological processes in aquatic and terrestrial ecosystems. The current model is based on a stationary collector for which predictions about capture efficiency (η; flux of captured particles : flux of particles) are based on the collector flow environment (i.e., collector Reynolds number, Rec; inertial : viscous forces). This model does not account for the movement of collectors in nature. We examined the effect of collector motion (transverse and longitudinal to the flow) on η using a cylindrical model in the lab and the grass species, Phleum pratense, in the field. Collector motion increased η (up to 400% and 20% in the lab and field, respectively) and also affected the spatial distribution of particles on collectors, especially at low Rec. The effect was greatest for collectors moving transversely at large magnitude, which encountered more particles with higher relative momentum. These results, which differ from the stationary model, can be predicted by considering both Rec and the particle dynamics given by the Stokes number (Stk; particle stopping distance : collector radius) and helped to resolve an existing controversy about pollination mechanisms. Collector motion should be considered in wind pollination and other ecological processes involving particle capture.