Near-Contact Motion of Surfactant-Covered Spherical Drops: Ionic Surfactant
✍ Scribed by Jerzy Bławzdziewicz; Vittorio Cristini; Michael Loewenberg
- Publisher
- Elsevier Science
- Year
- 1999
- Tongue
- English
- Weight
- 135 KB
- Volume
- 211
- Category
- Article
- ISSN
- 0021-9797
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✦ Synopsis
A lubrication analysis is presented for near-contact axisymmetric motion of spherical drops covered with an insoluble nondiffusing surfactant. The surfactant equation of state is arbitrary; detailed results are presented for ionic surfactants. The qualitative behavior of the system is determined by the dimensionless force parameter F ˆ, the external force normalized by the maximum resistance force generated by Marangoni stresses. For F ˆ> 1 drops coalesce on a time scale commensurate with the coalescence time 0 for drops with clean interfaces. For F ˆ< 1, the system evolves on the time scale 0 until Marangoni stresses approximately balance the external force; thereafter a slow evolution occurs on the Stokes time scale. In the long-time regime a self-similar surfactant concentration profile is attained that scales with the extent of the near-contact region. The gap width decreases exponentially with time but slower than for rigid particles because of surfactant backflow. For F ˆ< 1, drop coalescence does not occur without van der Waals attraction. Quantitative results depend only moderately on the surfactant equation of state.