Abstract: For process of an ultrathin surfactant-laden droplet spreading on preset film, base state and disturbance evolution equations for film thickness, surfactant interfacial and bulk concentrations were established with lubrication approximation. Transient growth analysis (TGA) was carried out to investigate stability of evolution process. Effects of surfactant solubility and typical parameters were discussed. TGA indicates that introduction of perturbation wave is conducive to enhance stability of droplet evolution and stability degree is directly related with wave number (k). However, stability tends to be deteriorated with increment of k. Compared with insoluble surfactant, evolution with soluble surfactant slows down slightly and stability is improved. Increasing of b, M, C, K_s stabilize the spreading, especially Marangoni effect and capillary force. Thickness of preset film has a primarily role on stability of droplet thickness evolution.

Key words: soluble surfactant, spreading, disjoining pressure, stability