Observation of gas flow around plants using Schlieren imaging system and high-refractive-index gas

The fruits of many plant are carried and flowers are also swayed by the wind. If the flow of air around plants can be visualized, the science behind it will be interestingly illustrated. In this study, the gas flow around cherry blossom fruits, clover flowers, maple seed propellers, and dandelion pappi as spherical and propeller-shaped samples is demonstrated using a Schlieren optical system and a high-refractive-index gas. The observed gas flow corresponding to the sample shape was well characterized by fluid dynamics features such as the Coandă effect. The results of experiments in which the flow of gas around plants is visualized are useful as a scientific education material for fluidics and optics.


Introduction
The Schlieren and shadowgraph methods are commonly used for visualizing fluids [1,2].Both methods clearly show changes in the refractive index of the medium through which light passes.Various fluids have been observed using these methods.Their use in shock wave, sound wave, and microfluid observations has been reported [3][4][5][6].The Schlieren and shadowgraph methods are suitable as observation methods in science education, and in some studies, they have been used as teaching tools [7][8][9][10][11].However, the observations using the Schlieren method often use artificial shape objects such as airplanes or only gas flow as a sample, although there are also examples of the application of these methods to the observation of living organisms.Liu et al reported the use of Schlieren photography of a flying hawkmoth [12].In addition, Gates used the Schlieren method to observe plants in research on heat transfer in plants [13].Cummins et al reported that a separated vortex ring underlies the flight of dandelion pappi [14].The author is interested in the naturally generated flow of gas around objects of various shapes exist close to us.The fruits of many plants are carried and flowers are also swayed by the wind.The author believes that if the flow of air around plants can be visualized, it will be interesting as a scientific demonstration method.
In this study, the gas flow around cherry blossom fruits, clover flowers, maple seed propellers, annual flebane flowers, and dandelion pappi used as spherical and propeller-shaped samples is observed using a Schlieren optical system and a high-refractive-index gas.

Experimental setup
For this experiment, a basic Schlieren optical system consisting of a pinhole, two lenses, and a knife edge was used as shown in figure 1. Figure 2 shows (a) an overview of the Schlieren optical system used in this experiment, (b) the pinhole and objective lens, (c) the knife edge, (d) the positional relationship between the sample and the spray can, and (e) the observed jet ejected from the spray can.A traditional incandescent bulb was used as a light source.Two objective lenses of the same astronomical telescope (Takahashi Seisakusyo Ltd, FC60-C) facing each other were used as the convex lenses of this Schlieren optical system.The aperture of these lenses was 60 mm and the focal length was 355 mm.The objective aperture of a scanning electron microscope made of molybdenum was used as a pinhole [15].The pinhole diameter used in this experiment was 0.35 mm.The knife edge was a craft knife blade (OLFA Co.), which was attached to the edge of the drawtube of the telescope with a double-sided tape.Therefore, the position of the knife edge in the optical axis direction can be manipulated in the same way as adjusting the focus of a telescope.All samples were placed between the two lenses.
The Schlieren optical system can visualize the refractive index gradient (dn/dy) of a gas such as air heated by the temperature of palm of a hand.The refractive index n of air is approximately 1.000 27 at room temperature and 1 atm.This refractive index changes by −1 ppm per 1 • C. Therefore, the refractive index change of air heated by body temperature is approximately −10 ppm.In this experiment, an easily available gas with a higher refractive index than air was used to facilitate observation.The stream applied to the sample was gas from a commercial spray used to blow away dust from a computer's keyboard.The gas in the spray can (Hozan Tool Industrial Co., Ltd, Z-283 134a Duster) was hydrofluorocarbon (HFC134a).The refractive index of this gas is approximately 1.0067.This gas, which has a higher refractive index than air, makes it easier to observe gas flows around samples.A sample was exposed to a gas flow from a direction perpendicular to the optical axis.In this experiment, plant fruits, flowers, seeds, and pappi were used as samples.The gas flow rate was measured with an anemometer (Sanwa Supply Inc., CHE-WD1).This instrument is a propellar-type anemometer and its minimum limit of flow rate detection is 0.4 m s −1 .In addition, figure 2(e) shows the observed jet ejected from the spray can.A thin tube (inner diameter, 1 mm) was connected to the tip of the nozzle of a spray can to generate a gas flow.As can be seen from this photograph, it was confirmed that the jet from the spray can be sufficiently clearly observed with this Schlieren optical system.In most of the following experiments, a gas was sprayed without the thin tube at a flow rate of about 1.4-2 m s −1 .

Gas flow around spherical sample
First, let us observe the gas flow around a spherical sample.Figure 3 shows the results of an experiment using a spherical fruit as an example of observing airflow around a spherical sample.Figure 3(a) shows a photograph of the Japanese Yoshino cherry blossom fruit used in this experiment, the shape of which is approximately    that the aggregate of inflorescence functions as a larger structure.

Gas flow around propeller-shaped structure
Next, the gas flow around a symmetrical propellershaped structure was observed using a Japanese maple (Acer palmatum) seed.Figure 5(a) shows a photograph of the propeller of a Japanese maple seed.The seed has a pair of winged samaras, each approximately 1.5 cm long, with seeds visible at their base.This photograph was taken in May.When the seed ripens in autumn, it flies as it rotates in the wind.Figures 5(b)-(d) show the flow of gas ejected from the nozzle and applied from below the seed.The gas that hits the lower part of the samara flows symmetrically along the outer circumference of the wings.This is also the Coandȃ effect.In addition, symmetrical flow patterns such as the 'U' shape appeared at the upper part of the samaras as shown in figure 5(d).Figure 5(e) shows the gas flow around a single samara.It can be seen that even a single wing is sufficient to affect the gas flow induced by the Coandȃ effect.

Gas flow around narrow gap structure
The flow of gas around another type of flower the was also observed.The flower of annual fleabane (Erigeron annuus) was used for this observation.the outer circumference of the flower.This is also the Coandȃ effect.It can also be seen that there is a relationship between the gap structure and the flow velocity such that the radial gas flow through gaps of less than 1 mm between ray florets can also be observed as shown in figure 6(d).

Gas flow around aggregate of fine fluffs
Finally, gas flow around a pappus was also observed.A pappus of the dandelion was used for this observation.Figure 7(a) shows a photograph of a pappus of the dandelion.Figure 7(b) shows an optical microscopy image of the dandelion pappus.It can be seen that among the approximately 100 main fluffs with a diameter of approximately 20 µm, there are short fluff-like branches.As shown in figures 7(c)-(g), various pattern of gas flow around the pappus can be observed.In this experiment, the gas was sprayed at a flow rate of less than 0.4 m s −1 .In figures 7(c)-(e), symmetrical flows were observed around the pappus.It can also be observed that the gas flow was affected by the pappus even in distant regions.Additionally, as shown in figures 7(f) and (g), an asymmetrical vortex formation can be observed when the gas was applied slightly from the side.From these observations, taking into account gas viscosity, it can be inferred that the aggregate of fine fluffs functions as a larger structure.

Discussion
The above experimental results revealed that plant fruits, flowers, and pappi have excellent properties as samples for use in the observation of the fluidic dynamics of gas flows around them.Since the above phenomenon is the behaviour of the flow during its relative motion toward the object, let us roughly estimate the Reynolds number.The Reynolds number is used to characterize various flows, such as laminar and turbulent flows.Laminar flows, in which viscous forces dominate, have low Reynolds numbers.On the other hand, turbulent flows are dominated by inertial forces and have high Reynolds numbers.Vortex profiles are associated with Reynolds number.The Reynolds number R e is expressed as follows.
where ρ is the density of the fluid, v is the speed of the fluid, L is a characteristic length, and µ is the dynamic viscosity of the fluid.For HFC134a, ρ is 30 kg m −3 and µ is approximately 12 µPa•s [18].In the experiment shown in figure 3, when v is approximately 2 m s −1 and L is 2 cm, the Reynolds number R e is estimated to be approximately on the order of 10 5 .In contrast, the example shown in figure 7 shows vortices.Owing to the reduced velocity (cm s −1 ) and L, the Reynolds number is expected to be below 1000, i.e. it is about a factor of 100 lower.When L is 1 cm, v is estimated to be approximately on the order of cm s −1 .This flow rate is considered reasonable considering that it could not be measured with an anemometer.Gas flow is observed in this system as a three-dimensional phenomenon.It was also found that the microstructures of plants act as a larger structure affecting gas flow, considering the viscosity of gas.The fact that gas flows around an object can be visualized even with such a simple experimental setup indicates that various objects can be used as observation targets.However, any HFC used in this experiment has-compared with chlorofluorocarbon (CFCs)-a small ozoneaffecting potential but a large greenhouse warming potential (GWP).The GWP of HFC-134a is 1300.Therefore, the use of other smaller GWP gases may be possible, for example, gases with a different refractive index such as Kr (n = 1.000 43 at 0 • C, 1 atm) and CO 2 (n = 1.000 45 at 0 • C, 1 atm) so this visualization experiment can be easily used in classrooms.Schlieren observations of plants of various shapes as described above will be useful for intuitively understanding the physical behaviour of gas flows around objects.Therefore, this experimental method using plants seems to be very suitable for scientific, technological, and engineering teaching demonstrations of, for example, optics and fluidics to stimulate the interest of students in science.
Next, let us discuss the optics.The optical system for this experiment included two telescopes, but magnifying glasses, camera lenses, or close-up lenses could also be used instead of two objective lenses.However, it is necessary to use a lens with little chromatic aberration.Single lenses have chromatic aberration.Therefore, monochromatic images are often observed using a bandpass filter when single lenses are used.Therefore, spherical mirrors without cromatic abberation are often used in Schlieren optical systems.These lenses of the astronomical telescope used in this experiment were apochromatic lenses.Therefore, chromatic and spherical aberrations are well corrected such that the focal spot diameter of the objective lens of this telescope is approximately 30 µm for plane waves (collimated light).Each colour of light from the C-line (656 nm) to G-line (430 nm) of Fraunhofer lines is focused within ±0.1 mm [19].Therefore, sharp Schlieren images were obtained as shown in the above mentioned figures.In this experiment, a pinhole with a diameter of 0.35 mm was used to ensure a sufficient amount of light.By using a pinhole with a diameter larger than the spot size, we can increase the tolerance for the spatial adjustment of the knife edge at the focal point by approximately 10 times, which also helps improve the operability of the optical system in experiments.

Conclusion
In this study, the gas flow around cherry blossom fruits, clover flowers, maple seed propellers, annual fleabane flowers, and dandelion pappi was demonstrated using a homemade Schlieren optical system and a jet from a spray can.The observed gas flow corresponding to the sample shape was well characterized by fluid dynamics features such as the Coandȃ effect.The author believes that the experimental results of visualizing the flow of gas around plants have a scientific impact and are useful as an educational tool for the scientific demonstration of fluidics and optics.

Figure 1 .
Figure 1.Schematic of Schlieren system used in this experiment.

Figure 2 .
Figure 2. (a) Overview of Schlieren optical system used in experiment, (b) pinhole and objective lens, (c) knife edge, (d) positional relationship between sample and spray can, (e) observed jet ejected from spray can.
Figure 3(d1) shows the flow of the gas upon hitting the centre of the sample, and figure 3(d2) is the same as figure 3(d1) with flow edges clarified by image processing.This photograph shows that the gas that hits the centre of the spherical sample flows symmetrically around the sample.Next, gas flows were observed around a nearly spherical flower.The flower used for this observation was a red clover (Trifolium pratense).

Figure 4 (
a) shows a photograph of a red clover flower.The flower has a dense inflorescence.Figures4(b) and (c) show the flow of gas ejected from the nozzle and applied from below the flower.The gas that hits the bottom of the nearly spherical flower flows symmetrically along the periphery of the flower.This is also the Coandȃ effect.Other symmetrical flows can also be observed at the upper part of the flower.The gas flow velocity shown in figure 4(c) was higher than that in figure4(b).From these observations, taking into account gas viscosity, it can be inferred

Figure 3 .
Figure 3. Observation of airflow around spherical plant fruit sample.Left: raw images and right: after image processing.(a) Yoshino cherry blossom fruit used in this experiment, (b1), (b2) gas flow when sample is not hit by gas, (c1), (c2) gas flow hitting bottom of sample, and (d1), (d2) gas flow hitting centre of sample.

Figure 4 .
Figure 4. Photographs of gas flow around red clover flower: (a) close-up image of red clover flower, and (b)-(c) Schlieren images of gas flow around red clover flower.

Figure 6 (
a) shows a photograph of annual fleabane flowers.The flower heads have many white ray florets.Figures 6(b) and (d) show the flow of gas ejected from the nozzle and applied from below the sample.When the gas collides with the lower part of the sample, it flows symmetrically along

Figure 5 .
Figure 5. Photographs of gas flow around winged samaras of Japanese maple: (a) close-up image of Japanese maple seed propellers, and (b)-(e) Schlieren images of gas flow around samaras.

Figure 6 .
Figure 6.Photographs of gas flow around annual fleabane flower [16]: (a) close-up image of annual fleabane flower, and (b)-(d) Schlieren images of gas flow around annual fleabane flower.Reproduced from [16].CC BY 4.0.