It is known that the traditional ‘nucleus’ theory for cavitation is incompatible with certain facts physically and experimentally recognizable in oil hydraulic flows. In order to re-examine this issue, cavitation of hydraulic oil flow through a long two-dimensional acrylic constriction was observed by various techniques: stroboscopic photography with a microscope, laser beam transmission, pressure and noise measurements, and luminescence and electrical charge detection. It was revealed that, at the incipient stage of cavitation, one stationary microscopic cavity always emerges suddenly on the inlet edge. Light emission was also observed in the cavitation together with electrostatic charge. All the findings taken together, which cannot be consistently explained by nucleus theory, lead to the following hypothesis: at the separation point a tensile force rips a liquid particle from the wall, leaving behind a vacuum space, which forms the incipient stationary cavity on the edge.

The effect of extracellular forces on the nucleus deformation is an important research issue for better understanding of the intracellular force transmission mechanism. Approaches to this issue employing a microtensile test of single cells are helpful because the test enables one to give a well-controlled load onto the specimen with wide force and strain ranges. In the present study, tensile tests of single cells having a spherical shape are conducted by using a microtensile test system with a feedback control of displacement rate. Deformations of the nucleus inside the cell during the cell stretch and subsequent creep recovery after unloading are then quantified based on an image analysis. In order to characterize the creep recovery behaviors of the cell and its nucleus, one-dimensional analytical viscoelastic models and a power-law function are fitted to the creep recovery data. In addition, systematic finite element analyses are performed to estimate the intracellular stress distribution and elastic modulus of the cell and nucleus assumed to be continuum materials. These results indicate that the mechanical behaviors of the nucleus within a cell under stretching and unloading are similar to those under compression loadings previously reported.

If bubble nuclei are the cause of cavitation, how are they initially produced? According to what Washio et al. have found out so far, there are two possible ways for cavitation nuclei to be generated in liquid flows: separation of flow and a relative motion between solids contacting in liquid. The present article intends to reinforce that assertion by observing the cavitation occurring in an oil hydraulic poppet valve. At a certain flowrate, a microscopic cavity suddenly emerged on the valve seat where the flow separated. As the flowrate increased, the cavity developed extending circumferentially on the seat and discharged bubbles by splitting. A collision of the poppet with the valve seat also caused the generation of a cavity. As the poppet was away from the seat after the collision, the cavity shrunk leaving behind a bubble. Cavities generated on the seat by flow separation regularly repeated a process of growth and shrinkage accompanied by bubble discharge, which induced flow pulsation and consequently vibration of the poppet supported by a spring as well. Moreover, these cavities brought about so-called ‘choking’ in the poppet—seat constriction and acted to increase the pressure loss there by narrowing its cross-section.

Cavitation occurring at a sharp projection in a hydraulic oil flow was observed in as much detail as possible, using, variously, a microscope, a high-speed video camera, laser beams, an electric charge detector and a photomultiplier. At the tip of the needle employed as a projection, a tiny cavity as small as several tens of micrometres in length suddenly emerged and would not go away. As cavitation became more vigorous, flashes occurred intermittently around the needle and positive electrical charges were generated which synchronized with the flashing. The electrode inserted downstream from the needle detected negative charge from the oil, which was also synchronous with the flashing. Observing the needle tip with the microscope made it possible to determine exactly the moment of cavitation inception, which turned out to depend on the oil temperature as well as the downstream pressure. All these findings tend to reinforce the ‘rip-off’ hypothesis previously proposed by the present authors for the cavitation mechanism.

Cavitation starting at the point of separation on a smooth cylindrical wall was observed. At the earliest stage of cavitation, a cavity suddenly emerged at the point of separation with its upstream tip attached to the wall. D iffering from incipient cavities arising at projected edges, the present one neither stayed nor formed a stable bubble on the wall. As it moved slightly downstream, the newly born cavity underwent severe deformation: i.e. it grew explosively, then split and ultimately vanished in reverse from the tail back, in as short a period as one ten-thousandth of a second. F lashing light was emitted from the cavity as it expanded, and a transient electric charge was also detected from the oil downstream in synchronization with this light emission. M oreover, a streamline extending from the point of separation was spontaneously visualized. In order to establish the reasons for this visualization, temperature distribution across the streamline was measured, revealing that heat has to be generated at the separation point. F inally, the mechanism of tensile stress and heat generation at the point of separation is theoretically discussed on the basis of Landau's analysis of a boundary layer ow along a separation line.