posted on 2015-01-13, 00:00authored byGovind Paneru, Bruce M. Law, Koki Ibi, Baku Ushijima, Bret N. Flanders, Makoto Aratono, Hiroki Matsubara
For hexadecane oil droplets at an
aqueous–air surface, the
surface film in coexistence with the droplets exhibits two-dimensional
gaseous (G), liquid (L), or solid (S) behavior depending upon the
temperature and concentration of the cationic surfactant dodecyltrimethylammonium
bromide. In the G (L) phase, oil droplets are observed to coalesce
(fragment) as a function of time. In the coalescence region, droplets
coalesce on all length scales, and the final state is a single oil
droplet at the aqueous–air surface. The fragmentation regime
is complex. Large oil droplets spread as oil films; hole nucleation
breaks up this film into much smaller fluctuating and fragmenting
or metastable droplets. Metastable droplets are small contact angle
spherical caps and do not fluctuate in time; however, they are unstable
over long time periods and eventually sink into the bulk water phase.
Buoyancy forces provide a counterbalancing force where the net result
is that small oil droplets (radius r < 80 μm)
are mostly submerged in the bulk aqueous medium with only a small
fraction protruding above the liquid surface. In the G phase, a mechanical
stability theory for droplets at liquid surfaces indicates that droplet
coalesce is primarily driven by surface tension effects. This theory,
which only considers spherical cap shaped surface droplets, qualitatively
suggests that in the L phase the sinking of metastable surface droplets
into the bulk aqueous medium is driven by a negative line tension
and a very small spreading coefficient.