Spontaneous epicuticular charging affects droplet dynamics on living leaves

TL;DR

These findings prove that electrostatic charging is a fundamental component of droplet-leaf interactions, opening new research directions from charge-affected leaf ecology to sustainable materials for droplet-based energy harvesting by tuning surface treatments and, moreover, opening new research directions from charge-affected leaf ecology to sustainable materials for droplet-based energy harvesting by tuning surface treatments.

Abstract

How water droplets move and slide on leaves influences plant ecophysiological and abiotic interactions, as well as the design of advanced bio-inspired wetting materials. Despite cross-disciplinary relevance, current descriptions of the in situ dynamics of droplets on living leaves focus almost exclusively on surface structure and chemistry, treating the leaf as a static, electrically neutral substrate. Here, three decades after the mechanistic discovery of the Lotus effect, we show that a yet 'hidden' force due to instantaneous electrical phenomena affect the dynamic droplet motion on living leaves. Using high-speed motion tracking and precision charge measurements, we show that droplets sliding on the pristine epicuticular wax layer on superhydrophobic Colocasia esculenta leaves strongly charge affecting its dynamics, previously observed only on synthetic (highly electronegative fluorinated) surfaces. Droplets accumulate charges of Qp,D1 = -0.02 to -0.15 nC per 30 uL droplet on pristine leaves. However, we specifically demonstrate the crucial role of the epicuticular wax layer plasticity: by a structural modification that decreases its roughness amplitude, the same leaves gain an impressive 30-40 fold enhancement in charge transfer (reaching Qt,D1 = -2.8 to -5.2 nC) slowing the droplet by half due to an estimated electrostatic force of 11 uN dominating the resistive forces. The charge accumulation is surface-history-dependent and charge quantities per droplet are surprisingly similar or even exceeding those recently reported from artificial surfaces. Our findings prove that electrostatic charging is a fundamental component of droplet-leaf interactions, opening new research directions from charge-affected leaf ecology to sustainable materials for droplet-based energy harvesting by tuning surface treatments and, moreover,...

Figures (17)

• Figure 1: Experimental setup and tracking of droplet sliding on pristine C. esculenta leaves. a) Schematic of the test setup in which a fresh leaf was fixed on a tilted antistatic sample holder enabling a homogenous sliding path of 40 mm, a droplet source was installed about 10 mm above the leaf and two electrodes ($E_1$ for neutralizing droplets before the slide and $E_2$ for measuring accumulated charges. A high speed camera operating at 1200 fps was used for tracking droplet dynamics. b) Photograph of a C. esculenta, the superhydrophobic leaves here have a length of 25-30 cm providing a sufficient area for the experiment. c) Photograph of a typical experiment: the leaf installed in the acquisition system, showing sliding path, droplet and electrodes. d) Sideview image recorded by the high-speed camera. Videos 1 and video 2 show the sliding motion and give an example of the tracking using an bounding box.
• Figure 1: Additional data of droplet dynamics measured across independent experiments on 9 different leaves. a) All measurements were conducted across four independent experiments (Experiment 1–9), with each one represented by a unique color to highlight experimental reproducibility under identical ambient conditions. The time axis starts at t=0 at the moment of first detection for each droplet. Data points represent every tenth measured value for clarity. b) Results for spatiotemporal droplet dynamics on treated leaves and higher droplet numbers comparing droplet D1 with D100, D200, D500, D900 showing that beyond D100 no significant furhter variation in the dynamics have been observed also for treated leaves.
• Figure 1: a) Evolution of bounding box loss during training. b) Training performance assessed by mean average precision (mAP) at Intersection over Union (IoU) thresholds 0.5 and 0.5 to 0.95.
• Figure 2: Droplet motion tracking on superhydrophobic pristine leaves. a) Total distance traveled by droplets at a given time on the same leaf and sliding zone. b) Higher detail analysis of droplet dynamics and total distance traveled by droplets D1-D100 showing a spatiotemporal evolution of droplet displacement with droplet number and slowdown of the first droplet.
• Figure 2: Surface roughness amplitude on pristine and treated leaves. AFM surface topography profiles have been recorded at the same position on a living Colocasia esculenta leaf, before (pristine, left panel) and after modification of the epicuticular wax layer (treated, right panel). This confirms surface smoothing of the micro-nano structure of the wax crystals through the gentle thermal treatment melting the wax crystals into a smoother layer as also suggested by the increased wettability of the treated surface.