“Shake Gels”: quantifying the role of the ions in the Hofmeister series

Abstract

This study explores the shear-induced reversible gel behaviour of suspensions containing silica nanoparticles and poly(ethylene oxide) (PEO). When subjected to vigorous shaking, these mixtures exhibit interesting shear thickening, transitioning from low-viscosity fluids to ‘shake-gels’ and then reverting back to liquid after some time. Rheological measurements conducted at constant shear rates reveal a significant increase in viscosity by several orders of magnitude occurring at specific time points during the process. Followed by that, lowering the shear rate is implemented to understand the relaxation behaviour. The investigation specifically examines the influence of various ions from the Hofmeister series on the gelation and relaxation dynamics of these suspensions. Results indicate that salts positioned on the right-hand side of the Hofmeister series, known as salting-in agents, facilitate quicker gelation and slower relaxation of the shake gels. Conversely, salts on the left-hand side, referred to as salting-out agents, lead to slower gelation and faster relaxation. These observations are attributed to the distinct effects that different salts exert on the conformation of PEO chains. Salting-in agents induce swelling of the polymer chains, enhancing their ability to interact and bridge between silica nanoparticles effectively, thus promoting rapid gelation and sustaining the gel structure for extended periods. In contrast, salting-out agents cause the polymer chains to collapse, reducing their bridging efficiency and resulting in delayed gelation along with a more rapid return to the fluid state during relaxation. Understanding the role of specific ions in modulating the gelation and relaxation behaviour of colloidal-polymer suspensions provides valuable insights for tailoring material properties in various applications, including drug delivery, oil drilling, environmental restoration, coatings, and soft matter engineering.