Research Topic

Laser Beam Interaction with Droplets: Computation and Measurement

Introduction

Optical droplet characterization provides an important tool for better understanding and optimization of numerous industrial processes involving droplets ad sprays, e.g. painting, spray cooling, spray drying, inhalators, aerosol sprays etc. The interpretation of light scattering from droplets is a pre-requisite to the advancement and improvement of existing measurement methods and to the development of new techniques.
In our project, we aim at (1) developing a proper theoretical model for light scattering by a deformed droplet in oblate shape, (2) testing the theory by experimentally measuring the two-dimensional scattering pattern from an acoustically levitated droplet, (3) developing new methods for droplet deformation and composition measurement according to the knowledge gained from (1) and (2).  

Theoretical Model:
From Geometrical optics, the scattered field can be calculated by the superposition of diffraction, reflection, and refraction (many orders).

Fig.1 Experimental Apparatus

Experimental Apparatus

Fig. 1 shows the schematic view of the apparatus. The acoustic levitator works at a frequency of 58 kHz. Due to the standing sound wave, the droplet is floated. By changing the power and/or the distance between transducer and reflector, we obtain droplets with different aspect ratios. The wavelength of laser is 632.8 nm. The first camera is used to take photographs of droplet. We use the second camera to detect rainbow pattern.


Experimental Results

Fig. 2 Light scattering patterns from spheroidal droplets in the rainbow region in order of increasing droplet aspect ratio (q=a/b).

Fig.2 shows the rainbow patterns for water droplet with refractive index 1.333. The aspect ratios are about 1.11, 1.19, 1.26, and 1.32 respectively. From Fig. 2, we can see that rainbow patterns are sensitive the droplet aspect ratio. The scattering from spheroidal drops of water into the rainbow region was observed to manifest hyperbolic-umbilic and cusp diffraction catastrophes. The scattering patterns are a generalization of the primary rainbow formed by a spherical droplet. Using GO method, Marston and others authors had found the relation between some special rainbow patterns and aspect ratio of spheroid droplet. Those patterns include transverse cusp, hyperbolic umbilic and E6 diffraction catastrophes etc. However, the relationship between all rainbow patterns and droplet aspect ratio has not been found.

The rainbow angle is relative to the refractive index of droplet. If we can detect the rainbow angle, then we could derive droplet refractive index of the droplet. The fringe frequency in equatorial plane depends on the size and refractive index. The opening angles and curvatures are also relative to the droplet parameters. So when we find these relationships, we could measure droplets including size, aspect ratio, and refractive index. We aim at finding these relationships.

Key Research Area

Multi-Physics, Data analysis

Contact

Haitao Yu
M.Sc.

Address:

Petersenstr. 32

D-64287 Darmstadt

Germany

Phone:

+49 6151 16 - 24401 or 24402

Fax:

+49 6151 16 - 24404

Office:

S4|10-106

Email:

yu (at) gsc.tu...

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