Browsing by Author "Saad, Shabab"

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    Diffusiophoresis of Active Colloids: From Synthesis to Dynamics in Complex Media
    (2020-04-28) Saad, Shabab; Natale, Giovanniantonio; Trifkovic, Milana; Benneker, Anne M.
    Active particles are examples of non-equilibrium systems that have gained a lot of momentum over the past few decades due to their ability to generate enhanced diffusive motion in fluid media. This work focuses on the synthesis and dynamics of active Janus colloids from single particle (dilute concentration regime) to cluster formations (concentrated regime). Two types of active colloids are explored: a) pH-responsive calcium carbonate (CaCO3) Janus colloids that are biocompatible in nature and b) silica particles half-coated with platinum Janus microspheres that are able to self-propel in presence of hydrogen peroxide. First, a methodology to half-coat the CaCO3 particles with a silica layer via Pickering emulsion is developed. Then the self-diffusiophoretic motion of the carbonate Janus colloids at different acid concentrations is investigated in simple Newtonian media. It is found that with increasing hydrogen ion concentrations, the pH-responsive colloids experience higher mean-square displacements due to self-propulsion velocities and longer particle trajectories. Synthetic Si/Pt active colloids were dispersed in two different viscoelastic fluids (Polyvinylpyrrolidone and Polyacrylamide) of different molecular weights and concentration regimes. These two systems were chosen to probe different relaxation times from relatively short (~5ms) for PVP to large (~14.5s) for PAM but always smaller than the rotary Brownian motion time scale (~20s). Within this regime, the coupling between the self-propulsion velocity and the medium rheology is investigated. The Janus colloids are found to get physically confined by polymeric entanglements but surprisingly they are able to escape the physical cage in a time scale much shorter than the relaxation time of the polymer solution. Finally we demonstrate how the collective motion of active colloids (cluster organization) can be autonomously controlled by tuning the fluid rheology. Such studies are highly relevant for applications of self-propelling colloids in targeted drug delivery, water and soil remediation where complex environments are naturally present.