Interfacial viscoelasticity in oscillating drops of cyclodextrin–surfactant aqueous solution: experiments and theory

We present experiments involving oscillating droplets in aqueous cyclodextrin–surfactant solutions. In these experiments, α-cyclodextrin (αCD) and anionic surfactants exhibit remarkable viscoelasticity at the liquid/air interface, with dilatational modulus varying across orders of magnitude. This rheological response depends on the concentrations of different complexes in the solution, particularly of the 2 : 1 inclusion complexes formed by two αCD molecules (αCD₂), and one surfactant (S). We propose a model that describes the distribution of these complexes on the droplet surface using a free energy approach, accounting for dipole–dipole interactions. The results of the model reproduce the interfacial behavior of the viscoelastic modulus and phase shift in excellent agreement with the data, clearly indicating that dipole–dipole interactions determine and control the viscoelastic properties of the drops.