Surface Friction Gradient-Mediated Directional Liquid Transport under Vibrational Activation.

This study introduces a method for directional liquid transport by leveraging surface friction gradients coupled with vibrational activation. We engineered friction gradients on polydimethylsiloxane (PDMS) substrates through controlled silicone oil infiltration, achieving significant variations in contact angle hysteresis. Surface characterization confirmed that higher silicone oil concentrations reduced hysteresis, enabling enhanced droplet mobility. Using a motorized vibrational platform, we systematically explored the interplay of vibration parameters, droplet size, and surface properties. High-speed imaging revealed that droplets exhibit asymmetric contact-line motion under optimized vibrational conditions, driving persistent migration toward higher-friction regions. The technique's versatility was demonstrated through guided droplet transport and controlled coalescence on dual-gradient substrates, with applications validated for merging droplets. These findings offer a robust framework for designing droplet manipulation systems.