Chutes Too Narrow: The Brazil Nut Effect and the Blessings of the Fall

Scientific papers written as dialogues evoke Platonist philosophical discourses and are foreseen as elementary forms of expression in neoclassical scientific renaissance. Here we report on a study of the Brazil nut effect in a series of macroscopic and microscopic systems in the form of a play in three acts. The nut effect predicting the segregation of smaller grains at the bottom of the mixture and larger ones at the top was observed in a polydisperse mix of manually shaken playground pebbles, albeit following different kinetics than in an agitated pile of chess pieces. Common sense predicts that vertical, gravity-driven segregation in colloidal dispersions with heterogeneous particle size distributions is such that larger particles would settle toward the bottom and the lighter ones cream at the top. Here we refute this expectation and demonstrate that segregation of colloidal particles in the vertical direction conforms to the Brazil nut effect when the agitation is delivered in the forms of vortex mixing or audio vibration, but not when the colloids are manually shaken. The effect is demonstrated on aqueous dispersions of hydroxyapatite and iron oxide nanoparticles. Kinetic experiments showed that segregation increases with the agitation time in mixtures of pebbles and with the post-agitation aging time in fine particle colloids. The particle size gradient established due to agitation-induced segregation led to a corresponding gradient in optoelectronic properties—band gap, conductivity, lattice acoustics—in the vertical direction of the colloidal column for both types of nanoparticles. Such morphological gradients induced by the Brazil nut effect can be harnessed as affordable and facile templates for the synthesis of functionally gradient materials. Discussion of the mechanism and the application potential of the observed effect in colloidal and coarse systems is enriched with a literature review of this curious effect and entwined with the literary elements of the dramaturgical narrative involving a family of scientist in exile from mainstream scientific institutions: the mother, the father, an elementary school boy and a kindergarten girl. The narrative conforms in its structure to the shape of a Brazil nut and presents the first research study whose findings are enacted through a dream. The play questions the humaneness and creative prospects of modern science and celebrates children’s science projects performed with meager resources and a plentiful of imagination, showing that they could be gateways to important scientific discoveries.