Surface Excess Energy as a Unifying Thermodynamic Framework for Active Diffusion.

Area of Science:

• Soft Matter Physics
• Chemical Engineering
• Materials Science

Background:

• Particle motion is usually driven by external gradients (chemical, electric, thermal).
• Chemical reactions can increase particle diffusion (active diffusivity) without external gradients, but mechanisms are not fully understood.
• Existing models like self-diffusiophoresis don't fully explain experimental observations.

Purpose of the Study:

• Investigate the fundamental mechanisms behind active diffusivity in catalytic Janus particles in reactive media.
• Quantify the contribution of interfacial reactions to particle motion beyond classical thermal limits.
• Develop a framework to explain and predict active diffusivity trends.

Main Methods:

• Studied catalytic Janus particles in reactive solutions without imposed gradients.
• Utilized both dissipative and nondissipative theoretical approaches.
• Assumed the surrounding aqueous bath remains near thermodynamic equilibrium.

Main Results:

• Interfacial chemical reactions generate excess surface energy and sustained interfacial stresses.
• These interfacial phenomena supplement thermal energy, leading to diffusion beyond the classical thermal limit.
• The developed framework accurately reproduces experimental trends, including non-monotonic diffusivity versus activity, for Janus particles and enzyme-driven vesicles.

Conclusions:

• Chemical reactions are a direct source of surface excess energy and surface-tension gradients, driving active diffusivity.
• This provides a new understanding of active matter mobility beyond phoretic effects.
• Offers design principles for controlling the motion of synthetic active particles.