Oscillatory motion of a camphor object emerging from nonlinear surface tension of deep eutectic solvents

Oscillatory motion of a camphor disk placed on a water phase containing hexylethylenediamine-glycerol, heptylethylenediamine-glycerol, or octylethylenediamine-glycerol as a deep eutectic solvent (DES) was investigated in terms of the nonlinearity between the surface tension and DES concentration, C_DES. The camphor disk is driven by the difference in the surface tension around it, therefore camphor dissolved from the disk is regarded as the energy source molecule [18, 19]. With an increase in C_DES, continuous motion changed to no motion via oscillatory motion, and the frequency of oscillatory motion decreased. The bifurcation concentration between continuous and oscillatory motion and that between oscillatory motion and no motion decreased with increasing alkyl chain length of the DES. The existence of a constant value of the surface tension, γ, at a lower C_DES and a decrease in γ at a higher C_DES, that is, the nonlinearity of the γ-C_DES curve, induces oscillatory motion and changes the motion features. The present study suggests that the characteristic features of self-propulsion can be created by the nonlinearity of the driving force derived from the combination of the energy source (camphor) and control (DES) molecules.