One way to focus the diffuse energy of a sound field in a liquid is by acoustically driving bubbles into nonlinear oscillation. A rapid and nearly adiabatic bubble collapse heats up the bubble interior and produces intense concentration of energy that is able to emit light (sonoluminescence) and to trigger chemical reactions (sonochemistry). Such phenomena have been extensively studied in bulk liquid. We present here a realization of sonoluminescence and sonochemistry created from bubbles confined within a narrow channel of polydimethylsiloxane-based microfluidic devices. In the microfluidics channels, the bubbles form a planar/pancake shape. During bubble collapse we find the formation of OH radicals and the emission of light. The chemical reactions are closely confined to gas-liquid interfaces that allow for spatial control of sonochemical reactions in lab-on-a-chip devices. The decay time of the light emitted from the sonochemical reaction is several orders faster than that in the bulk liquid. Multibubble sonoluminescence emission in contrast vanishes immediately as the sound field is stopped.cavitation | ultrasound | capillary waves

We present a study on achieving intense acoustic cavitation generated by ultrasonic vibrations in polydimethylsiloxane (PDMS) based microfluidic devices. The substrate to which the PDMS is bonded was forced into oscillation with a simple piezoelectric transducer attached at 5 mm from the device to a microscopic glass slide. The transducer was operated at 100 kHz with driving voltages ranging between 20 V and 230 V. Close to the glass surface, pressure and vibration amplitudes of up to 20 bar and 400 nm were measured respectively. It is found that this strong forcing leads to the excitation of nonlinear surface waves when gas-liquid interfaces are present in the microfluidic channels. Also, it is observed that nuclei leading to intense inertial cavitation are generated by the entrapment of gas pockets at those interfaces. Subsequently, cavitation bubble clusters with void fractions of more than 50% are recorded with high-speed photography at up to 250,000 frames/s. The cavitation clusters can be sustained through the continuous injection of gas using a T-junction in the microfluidic device.

The linear and nonlinear developments of Görtler vortices were experimentally investigated by means of hot-wire anemometer measurement. The wavelengths of the vortices were preset to be the most amplified using thin perturbation wires. Three different vortex wavelengths of 12, 15, and 20 mm were considered. These wavelengths were preserved downstream which confirm the prediction of the most amplified wavelength of Görtler vortices. The onset of the nonlinear region occurs at about the same Görtler number of 5.0 for all the wavelengths considered. In this nonlinear region, the secondary instability is initiated near the boundary layer edge, and it develops further downstream. The maximum turbulent intensity increases as the secondary instability becomes dominant in the flow. In the transition region, however, it slightly decreases before drastically increasing due to the onset of turbulence. Three maxima of intense turbulence are found in the turbulent intensity contours in the nonlinear region, which indicate the occurrence of the so-called varicose and sinuous modes of the secondary instability. Comparison with the previous available results shows that all data of maximum disturbance amplitude obtained from the same experimental setup seem to lie on a single line when they are plotted against Görtler number, regardless of the values of free-stream velocity and concave surface radius of curvature. Smaller radius of curvature results in higher vortex growth rate in the linear region due to stronger centrifugal effect. However, the vortex growth rate seems to be unaffected by free-stream velocity. The normal position of maximum disturbance amplitude reaches the maximum point exactly at the onset of nonlinear region before it drastically drops as the secondary instability is overtaking the primary instability.

The development of wall shear stress in concave surface boundary layer flows in the presence of Görtler vortices was experimentally studied by means of hot-wire measurements. The wavelengths of the vortices were preset by thin vertical perturbation wires so to produce the most amplified wavelengths. Three different vortex wavelengths of 12, 15, and 20 mm were considered, and near-wall velocity measurements were carried out to obtain the "linear" layers of velocity profiles in the boundary layers. The wall shear stress coefficient C f was estimated from the velocity gradient of the "linear" layer. The streamwise developments of boundary layer displacement and momentum thickness at both upwash and downwash initially follow the Blasius ͑laminar boundary layer͒ curve up to a certain streamwise location. Further downstream, they depart from the Blasius curve such that they increase at upwash and decrease at downwash before finally converge to the same value due to the increased mixing as a consequence of transition to turbulence. The spanwise-averaged wall shear stress coefficient C f , which initially follows the Blasius curve, increases well above the local turbulent boundary layer value further downstream due to the nonlinear effect of Görtler instability and the secondary instability modes. Three different regions are identified based on the streamwise development of C f , namely linear, nonlinear, and transition to turbulence regions. The onset of nonlinear region is defined as the streamwise location where the C f begins to depart from the Blasius curve. In the nonlinear region, the spanwise distribution of C f at the downwash becomes narrower, and there is no inflection point found further downstream.

Concave surface boundary-layer flows are subjected to centrifugal instability which results in the formation of streamwise counter-rotating vortices. Such boundary layer flows have been experimentally investigated on concave surfaces of 1 m and 2 m radius of curvature. In the experiments, to obtain uniform vortex wavelengths, thin perturbation wires placed upstream and perpendicular to the concave surface leading edge, were used to pre-set the wavelengths. Velocity contours were obtained from hot-wire anemometer velocity measurements. The most amplified vortex wavelengths can be pre-set by the spanwise spacing of the thin wires and the free-stream velocity. The velocity contours on the cross-sectional planes at several streamwise locations show the growth and breakdown of the vortices. Three different vortex growth regions can be identified. The occurrence of a secondary instability mode is also shown as mushroom-like structures as a consequence of the non-linear growth of the streamwise vortices. Wall shear stress measurements on concave surface of 1 m radius of curvature reveal that the spanwise-averaged wall shear stress increases well beyond the flat plate boundary layer values. By pre-setting much larger or much smaller vortex wavelength than the most amplified one, the splitting or merging of the streamwise vortices will respectively occur.