Capillary interactions between particles at liquid interfaces: From 2D particle monolayers to capillary suspensions
Vesselin N. Paunov
Department of Chemistry and Biochemistry, University of Hull, Hull, HU67RX, United Kingdom
We will discuss the capillary forces between small particles attached to a liquid interface. For freely floating particles this lateral force appears because the gravitational potential energy of the floating particles while the situation is quite different when the particles are partially immersed in a liquid layer on a substrate, where the effect is related to the particle wettability. Such lateral capillary forces can bring about the formation of a two-dimensional array (2D-colloidal crystals) from both micrometre-size and submicron particles. We will also consider the capillary bridge forces between particles dispersed in a fluid which can form capillary structured systems.
Recently, we produced capillary structured suspensions from hydrophilic particles suspended in a polar continuous media and connected by capillary bridges formed of minute amounts of an immiscible secondary (oil) phase . The capillary structuring of the suspension was made possible through local in-situ hydrophobization of the calcium carbonate particles dispersed in the polar media by adding very small amounts of oleic acid to the secondary liquid phase. We observed a strong increase in the viscosity of the calcium carbonate suspension by several orders of magnitude upon addition of such secondary oil phase compared with the same suspension without secondary phase or without oleic acid .
We also formed stimulus-responsive capillary-structured materials from hydrophobized calcium carbonate particles suspended in a non-polar phase (silicone oil) and bridged by very small amounts of a hydrogel as the secondary aqueous phase . Doping the secondary (aqueous) phase with methyl cellulose, which gels at elevated temperatures, gave capillary-structured materials whose viscosity and structural strength can increase by several orders of magnitude as the temperature is increased past the gelling temperature of the methyl cellulose solution. By using an aqueous solution of a low melting point agarose as a secondary liquid phase, which melts as the temperature is raised, we produced capillary-structured materials whose viscosity and structural strength can decrease by several orders of magnitude as the temperature is increased past the melting temperature of the agarose solution. The development of stimulus-responsive capillary suspensions can find potential applications in structuring of smart home and personal care products as well as in temperature-triggered change in rheology and release of flavours in foods and actives in pharmaceutical formulations.
 D.S. Dunstan, A.A.K Das, S.D. Stoyanov, P.S. Starck, V.N. Paunov, Langmuir, 2018, 34, 442-452.
 A.A.K Das, D.S. Dunstan, S.D. Stoyanov, P.S. Starck, V.N. Paunov, ACS Appl. Mater. Interf., 2017, 9, 44152–44160.