Adhesive force between paired microdroplets coated with lipid monolayers

Abstract

We created pairs of adhering water-in-oil microdroplets coated with lipid monolayers as model cells and studied the effects of the physicochemical properties of the lipids on the adhesive force ΔF. Four species of liquid-phase lipids were used: dioleoylphosphatidylethanolamine (DOPE), dioleoylphosphatidylcholine (DOPC), dipalmitoylphosphatidylcholine (DPPC), and dimyristoylphosphatidylcholine (DMPC). The dependence of ΔF on the choice of lipid was evaluated by independently measuring the interfacial tension at the oil–water interface, γ, and the contact angle between the adhering droplets, θ. It was found that a difference in size between the hydrophilic head and hydrophobic tail of the lipids results in an increase in γ. Hence, cone-shaped DOPE had a larger γ than did cylinder-shaped PC (γ: DOPE ≫ DMPC ∼ DPPC > DOPC). On the other hand, DMPC with the shortest tail length had the smallest θ among the lipids (θ: DOPC > DPPC > DOPE ≫ DMPC). Finally, it was found that ΔF drastically decreases when the carbon number of the alkyl chain in the tails is smaller than 16 (ΔF: DOPE > DOPC ∼ DPPC ≫ DMPC). Furthermore, using polyethylene glycol (PEG)-conjugated DOPE, we demonstrated that the conjugation of shorter PEG molecules (γ and ΔF become similar to those of the DOPC system.