Variable surface tension, due to the presence of surfactants or temperature gradient, occurs in a large variety of technologically important situations. It generally leads to tangential stresses along an interface between two fluids. The change in boundary conditions can have significant effect on the motion of buoyant bubbles and induce motion when buoyancy is small such as in microgravity.
Our work includes studies of the thermocapillary motion of bubbles and drops [1,2,3], the effect of insoluble surfactants on many bubbles [4,5], development of methods for soluble surfactants [6,7] and studies of the effect of soluble surfactant on motion of buoyant bubbles in channel flow .
 S. Nas and G. Tryggvason. “Thermocapillary interaction of two bubbles or drops.” Int’l J. Multiphase Flows 29 (2003), 1117–1135.
 S. Nas, M. Muradoglu and G. Tryggvason. “Pattern Formation of Drops in Thermocapillary Migration.” International Journal of Heat and Mass Transfer 49 (2006) 2265–2276.
 Min Lu, Jiacai Lu, Ying Zhang and Grétar Tryggvason. “Numerical study of thermocapillary migration of bubble in a channel with an obstruction.” Physics of Fluids. 31 (2019), 062101.
 Y.-J. Jan and G. Tryggvason, “Computational Studies of Contaminated Bubbles,” Symp on Dynamics of Bubbles and Vortices Near a Free Surface,” AMD Vol. 119 (Ed. Sahin and Tryggvason), pp. 46-59, ASME (1991). Y.-J. Jan’s doctoral dissertation has much more details that never made it into a journal paper.
 J. Lu, M. Muradoglu and G. Tryggvason. “Effect of Insoluble Surfactant on Turbulent Bubbly Flows in Vertical Channels.” International Journal of Multiphase Flow. 95 (2017), 135-143.
 M. Muradoglu and G. Tryggvason. “A front-tracking method for computation of interfacial flows with soluble surfactants.” Journal of Computational Physics 227 (2008), 2238-2262.
 M. Muradoglu and G. Tryggvason. “Simulations of Soluble Surfactants in 3D Multiphase Flow.” Journal of Computational Physics, 274 (2014), 737-757.
 Zaheer Ahmed, Daulet Izbassarov, Gretar Tryggvason, Metin Muradoglu, and Outi Tammisol. “Effects of soluble surfactant on lateral migration of a bubble in a pressure driven channel flow.” International Journal of Multiphase Flow 126 (2020) 103251 (12 pages)