Influence of brush fibers on sand ejection by a pendulum swept

¹ Department of Earth and Space Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka, 560-0043, Japan
² Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8601, Japan
³ Nagoya University Museum, Furo-cho, Chikusa-ku, Nagoya, 464-8601, Japan

^* e-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Published online: 1 December 2025

Abstract

Some birds rapidly move their wings or feet to toss sand into the air for the sake of self-maintenance by sandblasting. This behavior is called “dust bathing”. Although dust bathing is an important factor for the health of birds, its physical mechanisms have rarely been studied. Mechanics of sand tossing induced by a sweep of soft feather is intriguing also in terms of granular physics. In this study, we experimentally investigate the fundamental physical process of dust bathing. Specifically, a pendulum with a brush at its tip is used to sweep the surface of a sand bed, and the resulting sand ejection behavior is observed. In particular, we systematically vary the sweeping velocity v₀, sand particle size D_p, and measure the ejection mass M, the ejection speed V, and the ejection angle θ of the center of mass of splashed sand cluster. As a result, we find that M decreases with increasing v₀. However, V increases with increasing v₀. At a high v₀ regime, V is faster than v₀. The ejection angle θ gradually saturates as v0 increases. We also find that the ejection behavior (characterized by M, V, and θ) is independent of the sand particle size D_p under the experimental conditions of this study.