Co-operative motion of multiple benzoquinone disks at the air–water interface

Abstract

Self-motion of physical-chemical systems is a promising avenue for studying and developing mechanical functions with inanimate systems. In this paper, we investigate spontaneous motion of collections of solid macroscopic benzoquinone (BQ) disks at the air-water interface without intervention of chemical reactions. The BQ particles slowly dissolve and create heterogeneous interfacial tension fields on the water surface that drive the motion. Spontaneous, continuous locomotion was observed between multiple BQ particles, along with coupling, collisions, cycling and collective foraging for interfacial free energy. Analysis of the motion suggests co-operative behavior depends strongly on particle shape.

Self-propulsion of symmetric chemically active particles: Point-source model and experiments on camphor disks

“…The bifurcation is always continuous (no jump) but slightly above the threshold, the increase in velocity may be linear or involves a square-root singularity [Eqs. (19) and (27), respectively]. When the evaporation rate approaches zero, the threshold vanishes.…”

“…With the current interest in active matter, both autophoretic and interfacial swimmers have attracted attention as model systems to investigate collective and nonlinear effects, including clustering [24][25][26], oscillations, synchronization and pattern formation [5,27], density waves [28], swarming [29], and self-assembly [30]. Yet, in spite of a wealth of investigations [4,5,7], some questions remain on the mechanism of individual self-propulsion.…”

Role of Marangoni forces in the velocity of symmetric interfacial swimmers

“…The assumption of fixed concentration is indeed expected to be relevant in the numerous instances where the swimmer is entirely made of the surfactant itself. These include droplets [97] such as aniline oil and pentanol [101,102] or swimmers made of solid, such as benzoquinone, aspirin, or pure camphor [24,103,104]. Applying the same method as above and going back to the Stokes regime, we find that concentration swimmers with partial release do swim and their swimming velocity Pe * was obtained for different source radii (a s = 0.2, 0.4, 0.6).…”

“…First observations with small camphor scrapings moving spontaneously on water surface date back to the 18th Century [17,18] and as noted by Lord Rayleigh in 1890, the phenomenon, before being understood, "had puzzled several generations of inquirers" [19]. More than one century later, the interest in Marangoni swimmers was revived, this time focusing on the nonlinear and collective phenomena, such as synchronization, pattern formation and self-organization [20][21][22][23][24][25][26]. The advent of active matter [27] gave further impetus to the study of interfacial swimmers and they are by now under intense scrutiny [22,28,29].…”

“…(i) Our swimmer is not a hemisphere but an infinitely thin disk. This geometry is by far the most widespread [24,41,46,58,[64][65][66][67][68][69], presumably because it is most convenient to craft efficient-high velocity-swimmers. As we shall see, this seemingly minor difference can modify qualitatively some aspects of the swimming response.…”