Physical Ultrasonic Effects on Soils and Sediments

Physical Ultrasonic Phenomena. Besides chemical effects, cavitation from ultrasound also affects the physical surface of materials due to a variety of phenomena and thus is used in lysing cells, material synthesis, surface cleaning, particle filtration, and particle disaggregation/fracturing. Shockwave formation is dependent on the intensity of the cavitation collapse.

Acoustic streaming is dependent on the intensity of the sound source compared to the surrounding fluid. Microjets occur when a bubble collapses asymmetrically near a solid surface that is orders of magnitude greater in size than the cavitation bubble. Microstreaming is localized fluid flow due to the oscillating bubble. Each physical effect contributes to soil and sediment remediation, and promotes contaminant mass transfer.

Shockwaves are produced during cavitational bubble collapse when the velocity of the imploding cavitation bubble wall exceeds the speed of sound of the surrounding fluid or internal bubble gas. Shockwaves are pressure waves that nonlinearly propagate from the compressing bubble wall to the surrounding solution. Attaining velocities of 4000 ms^-1, shockwaves exceed the speed of sound in water. Shockwaves generate turbulent flow in homogeneous liquids. In heterogeneous solid-liquid systems, shockwaves accelerate particles causing particle-particle collisions. These high-velocity collisions smooth particle surfaces, and even cause colliding particles to simultaneously melt and agglomerate.

Acoustic streaming is the absorption of acoustic energy causing fluid flow velocities of 10cms^-1. Cavitation is not required to observe acoustic streaming. Increased ultrasonic power or frequency increases energy absorption. This creates a steeper energy gradient between the fluid affected by acoustic energy and the surrounding fluid, therefore increasing fluid flow rates. Figure 4 shows acoustic streaming in heterogeneous solid-liquid media, generating convective movement of bulk liquid.

Figure 4. Physical ultrasonic phenomena near solid surfaces (e.g. membranes and soil/sediment surfaces)

Microstreaming is small-scale and turbulent flow around the bubble surface due to bubble oscillations. This physical effect generates microscopic eddies that have an effective range of 1-100 pm around the bubble surface. If cavitational collapse occurs near solid surfaces, as shown in Figure 4, microstreaming brings fresh solution to solid surfaces, thereby solubilizing particles. Microstreaming promotes heat and mass transfer across solid-liquid interfacial films, acting as a key mechanism in surface cleaning.

Cavitational collapse near a solid surface produces another physical effect: microjets. As the bubble implodes, the fluid on the side opposite of the solid surface accelerates more rapidly compared to the fluid on the side of the surface. This difference causes an asymmetric bubble collapse, causing a high-speed fluid motion toward the solid surface. As depicted in Figure 4, microjets accelerate fluid perpendicularly toward the solid surface causing cavitational erosion and removal of insoluble contaminants. Numerical estimates of the jet velocity range from 100 to 200 m s^-1.