Phase-change-mediated transport and agglomeration of fungal spores on wheat awns.

Wheat and other staple crops are devastated by fungal diseases. Many fungal plant pathogens are spread via active or passive discharge of microscopic spores. Here, we described the unique transport of spores of the fungal pathogen Epicoccum tritici, causal agent of black sooty mould, on wheat awns. The unique multi-scale architecture of wheat awns, coupled with condensation and evaporation of dew droplets, facilitated the transport and agglomeration of spores of the fungus. First, dew droplets spontaneously transported spores from the tips of awn hairs to the neighbouring stomatal ridges, driven by gradients in Laplace pressure and surface wettability. Subsequently, spores agglomerated into dry clusters due to the Cheerios effect and evaporation, increasing the likelihood of passive spore removal via wind shear and/or rainsplash. Future plant breeding approaches should consider the development of modified spike structures, such as those without awns or awn hairs, to reduce the potential for spread of fungal plant pathogens.

Synergistic dispersal of plant pathogen spores by jumping-droplet condensation and wind.

Plant pathogens are responsible for the annual yield loss of crops worldwide and pose a significant threat to global food security. A necessary prelude to many plant disease epidemics is the short-range dispersal of spores, which may generate several disease foci within a field. New information is needed on the mechanisms of plant pathogen spread within and among susceptible plants. Here, we show that self-propelled jumping dew droplets, working synergistically with low wind flow, can propel spores of a fungal plant pathogen (wheat leaf rust) beyond the quiescent boundary layer and disperse them onto neighboring leaves downwind. An array of horizontal water-sensitive papers was used to mimic healthy wheat leaves and showed that up to 25 spores/h may be deposited on a single leaf downwind of the infected leaf during a single dew cycle. These findings reveal that a single dew cycle can disperse copious numbers of fungal spores to other wheat plants, even in the absence of rain splash or strong gusts of wind.

Thermoelectrics in ice slabs: charge dynamics and thermovoltages.

Thermoelectric effects of ice play an important role in many natural and engineering phenomena. We investigate, numerically and analytically, the electrification of finite-thickness ice slabs due to an imposed temperature difference across them. When exposed to a temperature gradient, thermoelectrification involves a fast initial stage dominated by Bjerrum defects and a subsequent slow stage driven by ionic defects. The time scales of the first and second stages are derived analytically and correspond to the Debye time scales based on the density of Bjerrum and ionic defects, respectively. For a given ice slab, at the steady state, the thermovoltage across it and the charge accumulation near its two ends depend strongly on its thickness, with the sensitivity of the thermovoltage being more pronounced. The discrepancy between the computed thermovoltage and experimental measurements is analyzed. The analysis shows that, although thermoelectric effects in ice were discovered 50 years ago, significant gaps, ranging from the bulk and interfacial properties of defects to the measurement of thermovoltage, exist in the quantitative understanding of these effects. Filling these gaps requires further experimental, theoretical, and computational studies.

Electrostatic Jumping of Frost.

The electrification of ice has been a subject of research since 1940, mostly in the context of charge generation in thunderstorms. This generation of electric charge is spontaneous, distinct from applying an external electric field to affect the diffusive growth of ice crystals. Here, we exploit the spontaneous electrification of ice to reveal a surprising phenomenon of jumping frost dendrites. We use side-view high-speed imaging to experimentally observe frost dendrites breaking off from mother dendrites and/or the substrate to jump out-of-plane toward an opposing polar liquid. Analytical and numerical models are then developed to estimate the attractive force between the frost dendrites and liquid, in good agreement with the experimental results. These models estimate the extent of charge separation within a growing sheet of frost, which is caused by mismatches in the mobilities of the charge carriers in ice. Our findings show that the unexpected jumping frost event can serve as a model system for resolving long-standing questions in atmospheric physics regarding charge separation in ice, while also having potential as a deicing construct.

Oil-Impregnated Hydrocarbon-Based Polymer Films.

Porous surfaces impregnated with a liquid lubricant exhibit minimal contact angle hysteresis with immiscible test liquids, rendering them ideal as self-cleaning materials. Rather than roughening a solid substrate, an increasingly popular choice is to use an absorbent polymer as the "porous" material. However, to date the polymer choices have been limited to expensive silicone-based polymers or complex assemblies of polymer multilayers on functionalized surfaces. In this paper, we show that hydrocarbon-based polymer films such as polyethylene can be stably impregnated with chemically compatible vegetable oils, without requiring any surface treatment. These oil-impregnated hydrocarbon-based films exhibit minimal contact angle hysteresis for a wide variety of test products including water, ketchup, and yogurt. Our oil-impregnated films remain slippery even after several weeks of being submerged in ketchup, illustrating their extreme durability. We expect that the simple and cost-effective nature of our slippery hydrocarbon-based films will make them useful for industrial packaging applications.