In this work, the interfacial phenomena of a single hanging drop have been observed and captured by a Schlieren optical system. The extraction fractions at different hanging times were determined. For the system without surfactant, the Marangoni effect induced by interphase mass transfer of a solute displays regular convection patterns. The addition of surfactants changed the mode of interfacial instability significantly but in different ways: SDS enhanced the mass transfer and Triton X-100 reduced the extraction fraction. (C) 2011 Elsevier Ltd. All rights reserved.