Immiscible surfactant droplets on thin liquid films: Spreading dynamics, subphase expulsion and oscillatory instabilities

► Immiscible surfactant droplets on thin liquid ('subphase') films are studied. ► The 'near-field' two-phase flow dynamics are characterized experimentally. ► The 'far-field' fingering instability is preceded by subphase trapping and release. ► Subphase expulsion is accompanied by an oscillatory contact line instability.