Abstract: Contact angle is the main indicator to measure the effect of droplets on the spreading and retention of rice leaves. In order to accurately measure the contact angle θ of droplets on rice leaf surface, influence of droplet volume on leaf contact angle of rice was studied. The leaf contact angle (θ), droplet height (h) and droplet contact surface diameter (d) of different droplet volumes were measured by experiment, and the variation of contact angle with droplet volume was analyzed. The results showed that droplet volume had a significant effect on leaf contact angle of the four tested rice. At the initial stage of the rise in droplet volume (about 1.0 to 7.5 μL), the ratio of rate at which the droplet height changes with droplet volume (h'_v) to the rate at which the droplet contact surface diameter changes with droplet volume (d'_v)>h/d, leaf contact angle of rice increased. In the later stage of the droplet volume continued to rise (7.5-20.0 μL), when h'_v/d'_v≈h/d, leaf contact angle of rice was basically unchanged. When h'_v/d'_v<h/d, leaf contact angle of rice decreases. Finally, the contact angle tended to be stable. It was concluded that the droplet volume had a significant effect on leaf contact angle of the rice, and the change trend of contact angle was closely related to the droplet height, contact surface diameter and growth rate of both. Finally, it was also proposed that the droplet volume used in the rice leaf contact angle experiment should be larger than the volume corresponding to the maximum contact angle of the rice leaf surface, and it was best controlled within the range of 7.5-20.0 μL. It was also concluded that the maximum contact angle of the rice leaf surface was within 7.5-10.0 μL of droplet volume, and it was approximated when h'_v/d'_v≈h/d for the first time.

Key words: contact angle, rice leaf, drop volume, drop height, droplet contact surface diameter