Leaves held droplets more firmly outdoors than in laboratory measurements
Droplets needed a steeper tilt to slide off leaves measured outdoors than after laboratory storage in an experiment on 30 evergreen plant species. The difference appeared in droplet retention, while the average contact angle showed no significant change between settings. Seasonal shifts also affected the readings. The peer-reviewed study highlights how sampling and handling shape measurements of water on foliage.
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The same leaves were measured outdoors and indoors
Water droplets needed a steeper leaf tilt to slide off during field measurements than during later laboratory measurements in a peer-reviewed experiment covering 30 evergreen broad-leaved species. The plants grew at Zhejiang A&F University in Hangzhou, China. Sampling in July and December 2025 included 19 tree species and 11 shrub species. Researchers selected five healthy plants per species and six sun-exposed leaves per plant, measuring the upper leaf surface immediately after detachment.[1], [2]
Before the indoor comparison, the leaf stalks were sealed and samples were transported in bags, then stored at 4 degrees Celsius for 12 hours. Laboratory measurements took place at 25 degrees Celsius. The sequence meant that storage, transport and measurement environment changed together. It therefore compared two handling protocols as well as outdoor and indoor conditions; the experiment did not isolate a single mechanism behind any difference.[1]
Droplet retention changed more than spreading
Contact angle describes how widely a droplet spreads on the surface. Researchers photographed a 5-microlitre droplet 30 seconds after placement. Retention angle instead records the tilt needed for a 50-microlitre droplet to slide; droplets still attached to a vertical leaf were assigned 90 degrees. Field retention angles averaged 11.7 degrees above laboratory values. Contact angle showed no significant average difference between settings, although individual species changed in opposite directions. Winter measurements were lower than summer measurements for both indicators.[1]
A broader dataset links the two measurements nonlinearly
Combining the experiment with five earlier studies gave a dataset spanning 161 species. Across that wider collection, contact and retention angles followed a curved rather than a straight-line relationship. A droplet’s resting shape and the ease with which it moves therefore provide different information about leaf–water interaction. One indicator cannot be assumed to describe every change visible in the other.[1]
The samples came from one humid urban location and included relatively few strongly water-repellent species. Water loss during storage, temperature and changes at the leaf surface are possible contributors discussed by the authors, with their individual effects unresolved. The work identifies season and sample preparation as important conditions for comparing how leaves retain or shed rainwater.[1]
