Non-Newtonian Behavior of Spherical Coacervate Micelle Dispersions in Aqueous Media

Non-Newtonian viscoelastic behavior of disordered spherical micelle dispersions composed of complex coacervate core micelles (C3Ms) in aqueous media is investigated by combining rotational rheometry and microfluidic particle focusing measurements. C3Ms were prepared by mixing oppositely charged block copolymers, resulting in spherical micelles with electrostatically neutral coacervate cores surrounded by hydrophilic coronas. Despite the spherical morphology and dispersion in low-viscosity solvents, lateral migration of micrometer-sized particles in microfluidic flow revealed measurable elasticity. The relaxation time (λ), quantified from particle focusing behavior, increased with micelle concentration and scaled with the short-time Brownian diffusion time scale. This suggests that viscoelasticity in C3M solutions originates from diffusive colloidal dynamics consistent with hard-sphere-like interactions. Upon salt addition, micelle dimension and λ decreased significantly, while solution viscosity remained nearly constant. These results demonstrate that λ can be a quantitative indicator of capturing micellar structural changes under external stimuli. This study highlights the use of centerline particle migration as a sensitive probe for weak viscoelasticity in micellar systems and establishes a quantitative link between micellar structure and relaxation dynamics.