Nucleation of cyclopentane hydrate by ice studied by morphology and rheology

Results from an experimental study of the effects of heterogeneous nucleation of cyclopentane hydrate by ice on the morphological properties of hydrate formed on a single aqueous drop immersed in cyclopentane and the rheological properties of density-matched 40% (v/v) aqueous fraction hydrate-forming emulsions are reported. Experimental observations indicate that the ice-oil-aqueous phase contact line is the hydrate heterogeneous nucleation site. A novel observation showing the critical nature of heterogeneous nucleation is reported: the heterogeneous nucleation of hydrate from ice is found to be qualitatively controlled by the temperature ramp rate in the experiment. Apparently, a low temperature ramp rate results in more heterogeneous nucleation sites for hydrate, and thus a higher surface area crystal structure than a rapid heating rate. The viscosity of a metastable hydrate-forming emulsion evolves rapidly when the emulsion is seeded with ice, although more slowly than when seeded with the hydrate itself. The critical time, defined as the time (measured from the seeding time) when an abrupt jump in viscosity is observed, decreases with higher subcooling; for hydrate-seeded emulsions this time varies from about 15 min to three hours as the subcooling is decreased from 7.3 °C to 4.3 °C; the slower growth with ice relative to hydrate seeding is seen in the larger critical time for ice seeding, as the critical time for ice seeding lags approximately one hour when compared to hydrate-seeded hydrate-forming emulsions, over this same range of subcoolings.