Differentiation of Back-and-Forth Motion in Self-Propulsion Emerging via Complex Formation

Area of Science:

• Physical Chemistry
• Surface Science
• Soft Matter Physics

Background:

• Self-propulsion of droplets is a key phenomenon in soft matter physics.
• Understanding droplet dynamics is crucial for applications in microfluidics and materials science.
• The influence of surfactants on droplet self-propulsion requires further investigation.

Purpose of the Study:

• To investigate the self-propulsion of thymol acetate (TA) droplets on sodium dodecyl sulfate (SDS) solutions.
• To elucidate the control effect of SDS concentration (C_SDS) on droplet motion.
• To analyze the driving forces and mechanisms behind the observed self-propulsion behaviors.

Main Methods:

• Experimental investigation of TA droplet motion on SDS aqueous solutions at pH 9.
• Measurement of surface tension of aqueous phases with and without thymol (TOH).
• Quantification of Marangoni flow speed around the droplet.

Main Results:

• Three distinct types of droplet back-and-forth motion were observed, dependent on C_SDS: partial oscillatory, full oscillatory, and full continuous motion.
• The partial mode exhibited a rest stage between displacements, while the full mode showed continuous motion.
• Changes in motion patterns correlated with variations in driving force and adsorption-desorption dynamics.

Conclusions:

• SDS concentration significantly controls the self-propulsion and motion patterns of TA droplets.
• Adsorption-desorption processes of TOH at the air/water interface, influenced by SDS, are critical for self-propulsion.
• Reactive systems with adsorption-desorption dynamics offer pathways to engineer droplet self-propulsion and deformation.