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.

Surfactant driven flows are simulated with three nonlinear partial differential evolution equations for film thickness, surfactant interracial and bulk concentrations based on lubrication theory via PDECOL program numerically, to investigate influence of Marangoni effect in positive, negative and neutral systems. Instability of droplet evolution in negative system is analyzed with method of transient growth analysis. It shows that Marangoni effect plays a positive role in promoting droplets non-uniform spreading and surfactant diffusion in a positive system. A system under negative Marangoni effect exhibits interfacial instability which resulted in spreading inhibited and violent fluctuations occurrence on droplet surface. Instability tends to be more obvious with strengthened effect. Marangoni stress is main driving force in transverse droplet evolution in both positive and negative systems.