Tunable Capillary Suspensions from Aqueous Two-Phase Systems

Adding small amounts of a (partially) immiscible fluid to a suspension can create liquid bridges between particles, leading to interconnected networks known as capillary suspensions. This can be used to structure suspensions and adjust their rheological properties. Typically, these suspensions involve water and oil, where the minority liquid phase wets the particles dispersed in the majority phase. Here, we have demonstrated that oil-free capillary suspensions can also be formed in aqueous two-phase systems (ATPS), where a phase separation occurs between two hydrophilic polymers, dextran and polyethylene glycol (PEG). In this system, silica particles form a self-standing gel when a small amount of the PEG-rich phase is added to the dextran-rich phase. Despite the ultralow interfacial tension in ATPS, a significant increase in storage modulus is achievable. Capillary bridges have been visualized using confocal microscopy. By adjusting the amount of the PEG-rich phase (secondary phase), the network strength and yield stress can be finely tuned, enabling a wide range of rheological responses. Due to the absence of oil and the use of hydrophilic, biocompatible polymers, these capillary suspensions have potential applications in biomedical (where living cells can act as particles), pharmaceutical, and food formulations, as well as in home and personal care products.