Uncontrolled fluid dynamics

The term rollover is used generally to describe the uncontrolled, spontaneous, penetrative convective self-mixing of two layers of multi-component cryogenic liquids with initially different densities. The fluid dynamics of this mixing phenomenon is incorrectly described by the word rollover. However, the term is in wide use today and is used in this monograph. Rollover is only one of a number of evaporation instabilities with cryogenic liquids, and is accompanied by a massive increase of evaporation rate over a long period of time (Mode A), measured in hours, or short period of time (Mode B), measured in seconds or minutes. 

Reality is often quite different. When a supercritical fluid mixture expands into pressures as high as ambient conditions, the resultant expansion plume can be a complex mixture it is a high velocity gas stream that entrains precipitated particles of extracted materials and often frozen carbon dioxide. Much adjustment needs to take place in the collection zone in order to achieve something close to 100 % recoveries of solutes with concentration ranges from parts per billion (PCBs) up to 50 % (total fat in a chocolate candy). Besides the flow dynamics of the expansion, several physicochemical parameters cause the deviation from the initial simple model. They include, but are not limited to, volatility of the solute, degree of co-precipitation of solid carbon dioxide (followed almost immediately with uncontrolled subhmation of the solid), aerosol formation, surface tension, occlusion in solid carbon dioxide, rebound from impinging surface, and many other interacting phenomena. 

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