Janttaky 20, 1922] 



SCIENCE 



75 



SPECIAL ARTICLES 



THE FORMS OF GAS AND LIQUID CAVITIES 



IN GELS, AND THEIR INTERPRETATION 



BY SURFACE COMPRESSION 



It has been observed by both chemists imd 

 biologists that gas bubbles arising in gels ex- 

 hibit lenticular forms. The most complete in- 

 vestigation of the phenomenon has been made 

 by Hatschek who, after making measurement 

 on many bubbles, endeavored by a statistical 

 study of their orientations to explain the ob- 

 served facts, including the divergence from 

 sphericity, by postulating definitely oriented 

 directions of cleavage within the gel, corres- 

 ponding presumably to a honeycomb micro- 

 structure of its water-poor phase. Neither 

 Hatschek himself nor later workers have been 

 convinced that this explanation was the true 

 one. Working with gelatine, but more especi- 

 ally with silica gels, the writers have produced, 

 from air-satm-ated media, controllable air- 

 bubbles both by rise of temperature and by 

 reduction of air pressure, and have observed 

 additional facts that lead to an altogether 

 different, albeit simple and complete, inter- 

 pretation of everything observed. 



Concomitantly as the gas content of a bubble 

 is caused progressively to diminish, the space 

 formerly occupied by the gas becomes filled 

 by infiltration with liquid from the liquid 

 phase of the gel, giving rise finally to liquid- 

 filled, phantomlike, cavities, whose very exist- 

 ence has heretofore escaped observation. The 

 forms of these cavities thus arising spontan- 

 eously in an isotropic medium on alteration of 

 a single external condition are exceedingly 

 symmetrical and beautiful. As demonstrated 

 by photomicrographs, they exhibit two main 

 types : 



(1) If derived from gas cavities of oblate 

 spheroidal form, the liquid-filled phantoms are 

 of forms that may be likened to bivalve mol- 

 luscs whose shells are either (a) segments of 

 spheres, or bowl-shaped; or (b) of inflected 

 curvature, like a circular basin with a flaring 

 edge; or (c) like the last, but with a central 

 apical spike like that of a helmet. Each one 

 of these forms is immediately explicable if it 

 be considered that, while forming, the original 

 airbubble thrust aside the elastic water-poor 



phase of the gel, which was thus obliged to 

 collect in an elastic layer or membrane under 

 compression round the periphery of the bubble. 

 The bubble cavity is thus contained and en- 

 veloped by a membrane which may appro- 

 priately , be considered in surface compression, 

 as contrasted with the customary surface ten- 

 sion, because adjacent portions of this en- 

 veloping membrane tend to move apart from 

 instead of toward each other, in a direction 

 tangential to the surface of curvature. The 

 sphere is the stable form that must be en- 

 veloped by a membrane in surface tension, 

 but is no longer stable if its bounding mem- 

 brane is in surface compression; and this in- 

 stability is, therefore, in such a case, relieved 

 flrst by an exhibition of oblateness and later 

 by an out-thrusting of the membrane in the 

 region of smallest radius of curvature, giving 

 rise to the forms observed in a manner en- 

 tirely predieable by purely geometrical rea- 

 soning. 



(2) If derived from gas cavities of pro- 

 late spheroidal form, the liquid cavities are 

 of forms somewhat like that of those walnuts, 

 occasionally met with, that have three instead 

 of two hmes or boat-like portions composing 

 their shells. This spontaneously formed solid, 

 trilunar, flgure has one axis of triad symmetry 

 perpendicular to one plane of symmetry, and 

 is usually of sharpened angle both along its 

 tlu'ee edge-ribs and especially at the ends of 

 its chief axis, by reason of the outward thrust 

 of its enclosing membrane, precisely as woidd 

 be predicted by the reasoning noted above. 

 An example of this form is shown in Fig. 2; 



FIG. 1 



while Fig. 1 shows the form referred to under 

 (b) above. These figures are from photo- 

 micrographs of cavities not more than one 

 millimeter in diameter; and each cavity con- 

 tains, besides liquid, a small spherical bubble 

 of air, which appears dark. 



