Boundary layer effects, ultrasound

The latest studies analyzing the mechanisms of contact ultrasound-assisted mass transfer have revealed that ultrasound-related cell disruption is limited to the thin layer of tissue that is in direct contact with the vibrating surface. In any tissue layers deeper than 1 mm, the structural changes that occur are attributed to water removal moreover, ultrasound can enhance such water removal as a result of mechanical effects that can loosen cell-boundary layers, and remove moisture from solid-liquid interfaces. The transport of moisture through the pore network can also be increased, and low-pressure cycles vhll enable an improved evaporation in the tissues. 

Ultrasonic particle manipulation can also be used to enhance biosensor performance, by using the ultrasound to drive the particles of interest to the biosensor surface. In this implementation (Fig. 5), a resonant reflector layer is used to generate a pressure node at a boundary, (effectively creating a quarter wave resonance within the fluid layer), rather than in the centre of the chamber, as previously described. 

Several theories aimed at explaining the phenomena have been proposed, each of which is founded on completely different concepts. Sripaipan et al. [21] proposed a nematic layer with free ends, in which the interaction between the longitudinal oscillations (induced by the motion of the free ends of the layer in compression) and the traverse oscillations establishes steady flow of the liquid and, as a result, rotation of the molecules. However, these authors used incorrect dispersion relations and their calculations are not consistent with observed layer compression patterns. Nagai and coworkers [26,27] hypothesized that with normal incidence of an ultrasound beam on the layer the rotation of molecules is attributable to radiation fluxes. Radiation fluxes are the steady acoustic flows caused by radiation forces in a traveling acoustic wave, the only provision being that the width of the ultrasound beam is smaller then the dimensions of the cell. In reality, radiation fluxes can only occur near the boundaries of the beam and produce a compression effect that is smaller than the one that is actually ob-... 

---