Convectional, sedimentation, and drying dissipative patterns of coffee in the presence of cream and in its absence

Abstract

Convectional, sedimentation, and drying dissipative structures of coffee with and without cream were studied on macroscopic and microscopic scales. Convectional pattern of colloidal particles of coffee with cream were clearly observed in this work and analyzed in a cup, a cover glass, a watch glass, and a glass dish. The convectional patterns were vigorous and irregular at the initial stage but soon highly distorted Bernard cells grew. The integrated total flows of the coffee particles coated with cream at the air–suspension interface were observed directly with the naked eyes from the central area toward outside edge at the initial stage (3 to 30 min), but the flow direction turned oppositely from the outside to the central area after 30 min. At the similar time, the short and few spoke lines appeared at the outside edge and grew long toward the central area. Then, the cooperative formation of clusters and bundles of the spoke lines took place at the middle and final convectional stages at the air–suspension surface, and then the dynamic sedimentation patterns appeared. The spoke lines of the coffee with cream were analyzed as a function of time elapsed. The drying patterns of coffee with and without cream were composed of the broad ring at the outside edge and a round hill accompanied sometimes with the spoke lines. These features are consistent with those of suspensions of nonspherical particles such as plate-like bentonite particles. The idea of the pinning effect is not supported, but importance of the gravitational and Marangoni convectional flows is clarified instead in this work.