The slip velocity of nearly neutrally buoyant tracers for large-scale PIV

The behaviour of nearly neutrally buoyant tracers is studied by means of experiments with helium-filled soap bubbles and numerical simulations. The current models used for estimating the slip velocity of heavy micro particles and neutrally buoyant particles are reviewed and extended to include the effect of unsteady forces and particle Reynolds number. The particle motion is analysed via numerical simulations of a rectilinear oscillatory flow and in the flow around an airfoil within a particle flow parameter space that is typical of large-scale PIV experiments. An empirical relation is obtained that estimates the particle slip velocity, depending on the particle-to-fluid density ratio, the particle Reynolds number and frequency of the local flow fluctuations. The model developed is applied to assess the slip velocity of helium-filled soap bubbles in a large-scale experiment conducted at the German–Dutch wind (DNW) tunnels in the flow around an airfoil, with chord Reynolds numbers up to three millions. Furthermore, a procedure is proposed that can be used to retrieve the bubbles mean density and dispersion from measurements of mean velocity and fluctuations, respectively. Graphic abstract

Soap bubbles seeding for quantitative time resolved velocity measurements of a turbulent wake flow behind a cylinder

The wake flow behind a cylinder of 100mm diameter is investigated using time resolved 2D PIV technique applied to an air flow generated in a closed loop open test section wind tunnel. The flow is seeded using a micro soap bubble generator (BG-1000, TSI Inc.). The bubbles in the air flow were illuminated with a CW laser source and imaged using a high-speed camera. The main purpose of this study is to show features and advantages of using soap bubbles as seeding for a relatively large-scale PIV investigation under low power illumination conditions.

Development of an Optical set-up for 3D PIV with a Large Volume

Measurements of 3D volumetric velocity fields are of great theoretical interest with numerous practical applications. These measurements are essential for studying volumetric flows that do not exhibit inherent flow symmetry, such as turbulence or vortex breakdown. In the past decade, several technological innovations facilitated the emergence of 3D-PTV techniques for measuring velocity fields at kHz rate with volumes of interest up to 104 cm3 that contain 300 µm helium-filled soap bubbles. However, when a commercial laser beam with millijoule pulse-energy is expanded and shaped to fill volumes above 102 cm3 for 3D-PTV experiments with 15 µm air filled soap bubbles, one finds that the power density of the laser source is insufficient to generate a signal image. This is because the power density of the laser beam falls inversely with respect to its cross-section area and due to the quadratic dependence of Mie-scattering on the particle diameter. Here, we report of the analysis and development of two optical techniques for extending the volume of measurement in volumetric PTV. In particular, when a volume about 103 cm3 is seeded with 15 µm air-filled soap bubbles and a laser with a pulse energy of few single mJ illuminates it. The first technique uses multi reflections between two opposing parallel mirrors. The second technique is a development of laser scanning PIV for volumetric scanning: The potential to increase the scanned volume is examined by experimenting with an acousto-optic modulator for fast scanning. Furthermore, by employing an off-axis parabolic mirror, we obtain parallel beam scanning, which increases the efficiency and quality of the scanning.

A novel volumetric velocity measurement method for small seeding tracers in large volumes

This paper presents the volumetric velocity measurement method of small seeding tracer with diameter 5µm ∼ 100µm for volumes of ≥ 500cm3. The size of seeding tracer is between helium-filled soap bubbles (HFSB) and di-ethyl-hexyl-sebacic acid ester(DEHS) droplets. The targeted measurement volume dimension is equivalent to the volume of HFSB, which will give a higher resolution of turbulence study. The relations between particle size, imaging and light intensity are formulated. The estimation of the imaging results is computed for the setup design. Finally, the methodology is demonstrated for turbulence velocity measurements in the jet flow, in which the velocities of averaged diameter 15µm air filled soap bubbles are measured in a volume of 7200cm3.

Control of Soap Bubble Ejection Robot Using Facial Expressions

This research has developed a soap bubble ejection robot as an amusement system that reads emotions from human facial expressions and controls the ejection of soap bubbles to improve human-robot interaction. A subject's response to soap bubble ejection is read by a built-in face recognition sensor which sends data to a control system which in turn controls the next ejection. Soap bubbles are often used to research children's emotions/emotional responses. First, evaluation experiments of the control system were performed using face photographs that show human emotions. The experimental results revealed that soap bubbles were ejected in the case of indifference, and the ejection stopped in the case of joy. Through the experimental results, it was confirmed that the control system worked properly when face photographs were used and also verified the effectiveness of the facial recognition sensor. Secondly, evaluation experiments were conducted with an actual human, and it was confirmed from the results that the control system operates as designed.

Physically Based Soap Bubble Synthesis for VR

To experience a real soap bubble show, materials and tools are required, as are skilled performers who produce the show. However, in a virtual space where spatial and temporal constraints do not exist, bubble art can be performed without real materials and tools to give a sense of immersion. For this, the realistic expression of soap bubbles is an interesting topic for virtual reality (VR). However, the current performance of VR soap bubbles is not satisfying the high expectations of users. Therefore, in this study, we propose a physically based approach for reproducing the shape of the bubble by calculating the measured parameters required for bubble modeling and the physical motion of bubbles. In addition, we applied the change in the flow of the surface of the soap bubble measured in practice to the VR rendering. To improve users’ VR experience, we propose that they should experience a bubble show in a VR HMD (Head Mounted Display) environment.

Turning plants from passive to active material: FERONIA and microtubules independently contribute to mechanical feedback

To survive, cells must constantly resist mechanical stress. In plants, this involves the reinforcement of cell walls, notably through microtubule-dependent cellulose deposition, and thus wall sensing. Several receptor-like kinases have been proposed to act as mechanosensors. Here we tested whether the microtubule response to stress acts downstream of known wall sensors. Using a multi-step screen with eleven mutant lines, we identify FERONIA as the primary candidate for controlling the microtubule response to stress. However, when performing mechanical perturbations, we show that the microtubule response to stress can be independent from FER. We reveal that the feronia phenotype can be partially rescued by reducing tensile stress levels. Conversely, in the absence of both microtubules and FER, cells swell and burst like soap bubbles. Altogether, this shows that the microtubule response to stress acts as an independent pathway to resist stress, in parallel to FER. We propose that both pathways are key components to turn plant cells from passive to active material.

Preppy Physics

Many interference phenomena that are seen in the natural world are characterized by alternating bands of pinkish-purple and aqua colors. These can be seen in oil films, soap bubbles, interference from scratches, supernumerary rainbows, glories, and other places where multiple orders of interference are present and the light source is broad band white light. It was such a clear manifestation of a common optical effect that it was very early on incorporated into computer simulation software. The effect also shows up in many basic optics experiments, such as on the edges of single-slit interference patterns. Why does white light multiple-order interference tend to produce aqua and pink colors?