Experimental observation of Shapiro-steps in colloidal monolayers driven across time-dependent substrate potentials

Area of Science:

• Condensed matter physics
• Soft matter physics
• Tribology

Background:

• Colloidal systems exhibit complex dynamics when driven over periodic substrates.
• Friction in these systems is influenced by substrate properties and external modulations.
• Dynamical mode locking can occur between driven systems and periodic potentials.

Purpose of the Study:

• To investigate the effect of time-periodic amplitude modulation on the driven motion of a colloidal monolayer.
• To analyze the emergence of Shapiro steps and their relation to friction reduction.
• To understand the discrepancy between experimental observations and numerical simulations regarding Shapiro steps.

Main Methods:

• Experimental setup involving a colloidal monolayer driven over a modulated substrate potential.
• Measurement of particle velocity as a function of driving force.
• Analysis of stick-slip dynamics and synchronization phenomena.

Main Results:

• A significant reduction in static friction force was observed with substrate modulation.
• A Shapiro step structure in the mean particle velocity was identified, indicating dynamical mode locking.
• Only a single Shapiro step was observed experimentally, unlike numerical simulations predicting multiple steps.

Conclusions:

• Time-periodic modulation effectively reduces friction and induces dynamical mode locking in driven colloidal systems.
• The formation of kinks in the colloidal crystal explains the limited number of observed Shapiro steps.
• Experimental findings highlight the role of collective behavior and defects in synchronization phenomena.