Crawling on directional surfaces

• We study model self-propelled crawlers exploiting directional frictional interactions.
• By directional we mean interactions sensitive to the sign of the sliding velocity.
• We provide formulae for the displacements arising from several types of shape changes.
• Reciprocal breathing modes are effective only when performed on directional surfaces.
• Our results are relevant to biological motility and for the design of crawling robots.

In this paper we study crawling locomotion based on directional frictional interactions, namely, frictional forces that are sensitive to the sign of the sliding velocity. Surface interactions of this type are common in biology, where they arise from the presence of inclined hairs or scales at the crawler/substrate interface, leading to low resistance when sliding ‘along the grain’, and high resistance when sliding ‘against the grain’. This asymmetry can be exploited for locomotion, in a way analogous to what is done in cross-country skiing (classic style, diagonal stride).We focus on a model system, namely, a continuous one-dimensional crawler and provide a detailed study of the motion resulting from several strategies of shape change. In particular, we provide explicit formulae for the displacements attainable with reciprocal extensions and contractions (breathing), or through the propagation of extension or contraction waves. We believe that our results will prove particularly helpful for the study of biological crawling motility and for the design of bio-mimetic crawling robots.