The chemical potential of water in the nectar drop, however, is not only determined by the concentration of sugar but also by the Laplace pressure resulting from its curvature and its interfacial tension. In the beginning, the difference of the osmolarity is the main driving force. However, at a later stage, when the difference of the osmolarity becomes small, the Laplace pressure would control the growth process. As the interfacial tension of the nectar drop is lowered due to the adsorption of the surfactant at the interface of the nectar drop and oil phase, the equilibrium size of the nectar drop with surfactant is expected to be higher than that without it.