Collective Motion Induced by Marangoni Instability in Spontaneous Oscillation of Surface Tension.

When a slightly soluble surfactant droplet is positioned at the tip of a capillary beneath the water surface, the surface tension begins to oscillate spontaneously─a phenomenon known as the spontaneous oscillation of surface tension (SOS). This behavior arises from transient Marangoni flows generated periodically by surfactant molecules. Our molecular-level observations reveal that this oscillatory behavior originates from periodic fluctuations in the surface pressure of the surfactant monolayer at the water surface, which are attributed to orientational changes in the surfactant molecules. We demonstrate that shortening the oscillation period requires increasing the diffusion and solubility of surfactants, while enhancing the amplitude depends on amplifying the momentum of the Marangoni flow produced by the surfactants. Furthermore, the elasticity of the surfactant monolayer critically determines the amplitude, while its dilational viscosity influences the time constant for surface tension recovery. These findings advance our understanding of SOS and offer insights into many-body systems driven by the Marangoni effect, such as cellular movement.