Tracking of Fluorescently Labeled Polymer Particles Reveals Surface Effects during Shear-Controlled Aggregation

Surface chemistry is believed to be the key parameter affecting the aggregation and breakage of colloidal suspensions when subjected to shear. To date, only a few works dealt with the understanding of the role of the physical and chemical properties of the particles' surface upon aggregation under shear. Previous studies suggested that surface modifications strongly affect polymer particles' adhesion, but it was very challenging to demonstrate this effect and monitor these alterations upon prolonged exposure to shear forces. More importantly, the mechanisms leading to these changes remain elusive. In this work, shear-induced aggregation experiments of polymer colloidal particles have been devised with the specific objective of highlighting material transfer and clarifying the role of the softness of the particle's surface. To achieve this goal, polymer particles with a core-shell structure comprising fluorescent groups have been prepared so that the surface's softness could be tuned by the addition of monomer acting as a plasticizer and the percentage of fluorescent particles could be recorded over time via confocal microscopy to detect eventual material transfer among different particles. For the first time, material exchange occurring on the soft surface of core-shell polymer microparticles upon aggregation under shear was observed and proved. More aptly, starting from a 50% labeled/nonlabeled mixture, an increase in the percentage of particles showing a fluorescent signature was recorded over time, reaching a fraction of 70% after 5 h.