High Velocity Interparticle Collisions Driven by Ultrasound

Abstract

During bubble collapse, intense shock waves are generated and propagate through the liquid at velocities above the speed of sound. [1][2][3][4] Unusual sonochemical effects are induced by these shock waves, most importantly, high velocity collisions among solid particles suspended in such liquids. 4 These collisions result in extreme heating at the point of impact, which can lead to effective local melting and dramatic increases in the rates of many solid-liquid reactions. [5][6][7][8] In this work, we describe a quantitative model of the melting induced by high-speed interparticle collisions and test this kinematic model against the effects of varying initial particle size and slurry concentration on the morphology of zinc particle agglomerates.Sonication 9 of a decane slurry containing 2% w/w fine Zn powder (5 µm diameter) rapidly produces Zn agglomerates (cf. scanning electron micrographs in Figure 1 and Supporting Information). As sonication proceeds, agglomeration reaches its maximum effect after ∼90 min. The resulting 50-70 µm agglomerates have nearly round shapes ( Figure 1B). Sonication of 5 µm Zn powder as a slurry in alkanes, for example, produces dense agglomerates consisting of ∼1000 fused particles.Because of turbulent flow and shock waves generated by cavitation in liquids irradiated with ultrasound, metal particles are driven together at extremely high speeds, which induces effective melting at the point of impact. 4 The estimated velocity of colliding particles approaches half the speed of sound in the liquid. 4 The low melting point of Zn (419.6°C) 10 obviously contributes to the facile agglomeration process. One would expect to alter the velocity of interparticle collisions by varying the concentration of impinging particles. This should influence the temperature at the site of impact, resulting in agglomeration with diminished efficacy at sufficient slurry density. To verify this effect, the slurry loading was systematically increased. Loadings up to 50% w/w showed no significant effect, but further increases to 70% w/w resulted in considerably less pronounced agglomeration ( Figure 1C and Supporting Information).The particle size has a very strong effect on the outcome of the ultrasonic irradiation. From previous observations, 8 we know qualitatively that agglomeration does not occur for particles either too large (∼100 µm) or too small (∼100 nm). For example, no aggregation was observed for coarse Zn powder (Figure 2), although particle deformation does occur (Supporting Information). Interestingly, by mixing the fine and coarse Zn powders and sonicating them together as a slurry at high loading, a porous aggregated product is formed ( Figure 2B). The large particles are literally welded together by collision with the smaller particles.To model the interparticle collisions, some simplifying approximations will be made: (1) the collisions are perfectly inelastic (i.e., all kinetic energy ends as thermal energy within the particle colliding) and (2) complete melting of a particle occurs in ord...

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