Adhesive force between paired microdroplets coated with lipid monolayers

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. 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 among the lipids (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 (<750) to the head part of the DOPE changes its molecular shape to cylindrical, and thus its γ and ΔF become similar to those of the DOPC system.