Sporopollenin capsules at fluid interfaces: particle-stabilised emulsions and liquid marbles

Hollow particles of sporopollenin from spores of the plant Lycopodium clavatum have been prepared. These particles possess ionisable groups on their surface such that they become increasingly negatively charged with an increase in pH. The particles adsorb to both air-water and oil-water interfaces, stabilising liquid marbles and emulsions respectively. Water marbles can be formed at all pH values between 2 and 10 and for salt concentrations up to 1 M NaCl. Their stability to evaporation is greater than the equivalent bare water drop. Deformation and buckling of the marbles occur as water is lost due to the solid-like nature of the liquid surface. Transitional phase inversion of emulsions occurs from water-in-oil to oil-in-water with increasing pH, with that corresponding to inversion depending on the oil type. Emulsion drops of millimetre size are stable to coalescence due to a close packed layer of particles at their surfaces.