Kosei Takehara

The air bubble entrapped under a drop impacting on a solid surface

We present experimental observations of the disk of air caught under a drop impacting onto a solid surface. By imaging the impact through an acrylic plate with an ultra-high-speed video camera, we can follow the evolution of the air disk as it contracts into a bubble under the centre of the drop. The initial size and contraction speed of the disk were measured for a range of impact Weber and Reynolds numbers. The size of the initial disk is related to the bottom curvature of the drop at the initial contact, as measured in free-fall. The initial contact often leaves behind a ring of micro-bubbles, marking its location. The air disk contracts at a speed comparable to the corresponding air disks caught under a drop impacting onto a liquid surface. This speed also seems independent of the wettability of the liquid, which only affects the azimuthal shape of the contact line. For some impact conditions, the dynamics of the contraction leaves a small droplet at the centre of the bubble. This arises from a capillary wave propagating from the edges of the contracting disk towards the centre. As the wave converges its amplitude grows until it touches the solid substrate, thereby pinching off the micro-droplet at the plate, in the centre of the bubble. The effect of increasing liquid viscosity is to slow down the contraction speed and to produce a more irregular contact line leaving more micro-bubbles along the initial ring.

Experiments on bubble pinch-off

A bubble is slowly grown from a vertical nozzle until it becomes unstable and pinches off. We use ultra-high-speed video imaging, at frame-rates up to 1millionfps, to study the dynamics and shape of the pinch-off neck region. For bubbles in water (Bo≃1.0) the radius of the neck reduces with a power law behavior R∼tα, over more than 2 decades, with an exponent in the range α=0.57±0.03, in good agreement with other available studies, but which is slightly larger than 1∕2 predicted by Rayleigh-Plesset theory. The vertical curvature in the neck increases more slowly than the azimuthal curvature, making the neck profiles more slender as pinch-off is approached. Self-similar shapes are recovered by normalizing the axial coordinate by a separate length scale which follows a different power law, Lz∼tγ, where γ=0.49±0.03. Results for air, He, and SF6 gas are identical, suggesting that the gas density plays a minimal role in the dynamics. The pinch-off in water leaves behind a tiny satellite bubble, around 5μm in d...

The initial coalescence of miscible drops

When two drops of different miscible liquids are brought into contact, their coalescence speed is governed by the liquid having the weaker surface tension. Marangoni waves propagate along the drop with the stronger surface tension. We present surface profiles and propagation speeds of these waves, from experiments with a pendent water drop coalescing with a flat ethanol surface or with a sessile drop of ethanol. We find in the former case that the capillary-Marangoni waves along the water drop show self-similar character when measured in terms of arc length along the original surface. The coalescence of two liquids of different viscosities is also studied. For large viscosity difference, mobility is confined to the lower viscosity fluid and a sharp corner forms where the two liquids meet along the free surface. The coalescence speed of a water drop with a much more viscous liquid is nearly independent of the strength of the viscosity difference.

Crown breakup by Marangoni instability

We present experimental observations of hole formation in ejecta crowns, when a viscous drop impacts onto a thin film of low-viscosity liquid with significantly lower surface tension than the drop liquid. The holes are promoted by Marangoni-driven flows in the sheet, resulting from a spray of fine droplets ejected from the thin film hitting the inner side of the crown. The puncturing of the sheet takes place in three distinct steps. First a circular patch of the sheet thins by Marangoni-driven flows. Then this thinner film ruptures and a hole quickly opens up spanning the patch. Finally, the hole opens up further at an accelerated rate, driven by the unbalanced surface tension at its edge. The holes grow until they meet adjacent holes, thus leaving a foam-like network of liquid filaments, which then breaks up into a cloud of droplets.

Microjetting from wave focusing on oscillating drops

We present experimental observations of microjetting from an oscillating drop. The jet is generated by the focusing of axisymmetric capillary waves that overturn and collide at an apex of the drop. These jets are up to two orders of magnitude smaller than the original drops. We present two widely different configurations that produce such microjets. The first occurs on a satellite drop, produced by the pinch-off of a water drop from a vertical nozzle. The large oscillations following the contraction of the satellite bridge focus waves at the bottom, sending out a 30μm jet at 9.9m∕s. The second jet arises when a water drop, containing surfactants, falls onto and passes through a hemispherical soap film. The gentle deformation of the drop creates a surface wave that focuses at its top, shooting out a tiny jet and entrapping a small bubble inside the drop. This jet is 16±5μm in diameter and emerges at 6.3m∕s. In this configuration, the soap film wraps around the drop and acts as a sensor of the air flow, rev...

The offshore vortex train

A row of two-dimensional vortices forms in an offshore zone when regular surface waves run up a sloping flat bed. This vortex row is called the offshore vortex train. The vortices begin to appear near the breaking point. Moving in the offshore direction, they develop and increase their horizontal lengthscale through vortex merging. After reaching a particular offshore location, however, they decay rapidly. The formation region of the vortex train has been investigated on the basis of visual experiments for three bed slopes. Its formation does not depend on the type of wave breaking but is observed when the steepness of deep-water waves is smaller than 4.2×10 -2 . The horizontal lengthscale of the vortices and the velocities of the vortex movement have also been evaluated empirically

Visualization of a droplet falling on the water surface

Surface deformation by impact of a droplet and entrainment of the droplet into water arc visualized by means of a high-speed video camera developed by the authors, of which framerate is 4, 500 pps. The average diameter of droplets used in the experiment is 4.6 mm. The height of the nozzle which releases droplets is varied in the range of 5-45 cm.Some preliminary results are obtained as follows : (1) The relation between height (i.e., impact speed) and size of the crater changes for the low or high falling velocity, which implies the process is also different, (2) The surface deformation and entrainment processes can be clearly visualized in detail by the camera.

A 100 Mfps image sensor for biological applications

Two ultrahigh-speed CCD image sensors with different characteristics were fabricated for applications to advanced scientific measurement apparatuses. The sensors are BSI MCG (Backside-illuminated Multi-Collection-Gate) image sensors with multiple collection gates around the center of the front side of each pixel, placed like petals of a flower. One has five collection gates and one drain gate at the center, which can capture consecutive five frames at 100 Mfps with the pixel count of about 600 kpixels (512 x 576 x 2 pixels). In-pixel signal accumulation is possible for repetitive image capture of reproducible events. The target application is FLIM. The other is equipped with four collection gates each connected to an in-situ CCD memory with 305 elements, which enables capture of 1,220 (4 x 305) consecutive images at 50 Mfps. The CCD memory is folded and looped with the first element connected to the last element, which also makes possible the in-pixel signal accumulation. The sensor is a small test sensor with 32 x 32 pixels. The target applications are imaging TOF MS, pulse neutron tomography and dynamic PSP. The paper also briefly explains an expression of the temporal resolution of silicon image sensors theoretically derived by the authors in 2017. It is shown that the image sensor designed based on the theoretical analysis achieves imaging of consecutive frames at the frame interval of 50 ps.

A bright and long-pulse illumination for ultrahigh-speed microscopy of living specimens

Ultrahigh-speed microscopy of living specimens requires ultrabright illumination. Moreover, the duration of illumination should be sufficiently long, on the order of at least several tens of milliseconds, in order to investigate the dynamic state of living specimens. However, specimens are exposed to a high risk of damage by the intense illumination. The brightness and pulse duration of illumination have to be continuously controlled for use in the ultrahigh-speed microscopy of living specimens. Commercial or laboratory-made illumination systems do not satisfy the abovementioned requirements. In this paper, the development of a bright and long-pulse illumination system for ultrahigh-speed microscopy of living specimens is presented. A xenon flashlamp with an arc length of 1.5 mm has been used as the light source. The electrical power supply consists of a voltage-regulated circuit, a capacitor bank, and a control circuit including an insulated-gate bipolar transistor as a gating device, which provides a large rectangular current pulse with the duration in the range to the order of several tens of milliseconds. The brightness, pulse duration, and repetition rate can be easily and continuously controlled. The illumination developed in the present study is installed in an inverted fluorescence microscope equipped with a high-speed camera in order to evaluate the performance as an illumination source. A fluorescent image of the living spermatozoa of a mouse obtained at a frame rate of 8 kHz shows good contrast. Such an image cannot be obtained using a commercial illumination system.

In search of techniques to obtain dynamic fluorescence images of cellular phenomena occurring at ultra-high speed

Although we have aspired to observe dynamic changes in fluorescent images at the cellular level for a long time, the commercially available video cameras are not at all suitable for this purpose because of their low frame rates and photosensitivity. The present work tackles this issue and describes our attempt to find a solution by using our high-speed video camera and an ultrabright illumination system. We used light sources with considerably higher energy because conventional mercury lamps cannot produce sufficient brightness for our video cameras working a rate of more than 4,500 fps to obtain fluorescent images of cells. We observed that the flagellar movement of mice sperms ceased and multiple kinks developed in their tails when exposed to 2.7W of laser illumination for 1 s. In contrast, no significant alterations could be detected when the sperms were subjected to the same amount of energy by intermittent illumination. Since we found that cells can survive short-duration exposure to high-energy light, we attempted to construct an ultrabright Xenon-strobe illumination system. Our fluorescence studies are currently being extended to other types of animal cells, e.g., observation of the conduction of action potentials in the peripheral nerves of frog.