Motion and Interaction of Aspirin Crystals at Aqueous–Air Interfaces

Abstract

Small-molecule amphiphiles such as aspirin have unique properties arising from a combination of an aromatic hydrophobic part and a hydrophilic part. We show that crystals of aspirin are capable of generating convective flows at the air-aqueous interface from both Marangoni effects (through weak surface activity) and capillarity (surface deformations). The flow-driven motion of millimeter-sized crystals was found to depend on the presence of other ions in solution as well as the distance and orientation of the crystals. The interactions lead to the formation of groups of two or more crystals that also underwent motion. The convective flows created by small amphiphile crystals might be exploited in the dynamic self-organization of particles at interfaces.

Evolution of Self‐Propelled Objects: From the Viewpoint of Nonlinear Science

“…Thus, R-(COOH) 2 provides the driving force of selfmotion rather than R-(COO À ) 2 .A saresult, ac amphoric acid disk/boat exhibits continuous motion at low HPO 4 2À concentration, and oscillatory motion at high HPO 4 2À concentration. [33] [89,90] In the case of an aspirin disk (R''-COOH, R'' = C 8 H 7 O 2 ), oscillatory motion was observed in NaOH aqueous solution with high pH ( % 11).…”

“…For example, ah ematic catalystp article covered with polymer decomposesH 2 O 2 under blue light illumination, and therefore the clusters are formed only with illumination. [33,36,[143][144][145] In the case of camphor disks floating on water, there are two sources of interaction:s urfacet ension (repulsive interaction) and hydrodynamic interaction that can work as either repulsive or attractive depending on the distance between the disks and the depth of the water phase. Another example is chemical stimulus.…”

“…Another example is chemical stimulus. [33,145] In addition, it is reported that self-propelled droplets can arrange their swimming directions though hydrodynamic interaction. After the NH 3 is removed by evaporation,t he particles start to form clusters, in which electroosmotic flow will be the dominant interaction term even thought he phoretic effect also providess ome opposite interaction.…”

“…[89,90] In the case of an aspirin disk (R''-COOH, R'' = C 8 H 7 O 2 ), oscillatory motion was observed in NaOH aqueous solution with high pH ( % 11). [33]…”

“…[141] Self-propelled objectsd riven by interfacial tension can also form clusters in which the objectsd on ot contact each other, and the spacingb etween the objects is larger than the object size. [33,36,[143][144][145] In the case of camphor disks floating on water, there are two sources of interaction:s urfacet ension (repulsive interaction) and hydrodynamic interaction that can work as either repulsive or attractive depending on the distance between the disks and the depth of the water phase. [36,[143][144][145] As the result of balance between those interactions, camphor disks can form characteristico rdered structures.…”