  Hypothesis: Particle-laden fluid interfaces are the central component of many
natural and engineering systems. Understanding the mechanical properties and
improving the stability of such interfaces are of great practical importance.
Janus particles, a special class of heterogeneous colloids, might be utilized
as an effective surface-active agent to control the assembly and interfacial
rheology of particle-laden fluid interfaces. Experiments: Using a custom-built
interfacial stress rheometer, we explore the effect of Janus particle additives
on the interfacial rheology and microscopic structure of particle-laden fluid
interfaces. Findings: We find that the addition of a small amount of
platinum-polystyrene (Pt-PS) Janus particles within a monolayer of PS colloids
(1:40 number ratio) can lead to more than an order-of-magnitude increase in
surface moduli with enhanced elasticity, which greatly improves the stability
of the interface. This drastic change in interfacial rheology is associated
with the formation of local particle clusters surrounding each Janus particle.
We further explain the origin of local particle clusters by considering the
interparticle interactions at the interface. Our experiments reveal the effect
of local particle structures on the macroscopic rheological behaviors of
particle monolayers and demonstrate a new way to tune the microstructure and
mechanical properties of particle-laden fluid interfaces.
