Research Papers: Multiphase Flows

Effect of Liquid Transparency on Laser-Induced Motion of Drops

[+] Author and Article Information
R. Shukla

School of Mechanical and Aerospace Engineering, Oklahoma State University, Stillwater, OK 74078

K. A. Sallam1

School of Mechanical and Aerospace Engineering, Oklahoma State University, Stillwater, OK 74078khaled.sallam@okstate.edu


Corresponding author.

J. Fluids Eng 131(8), 081301 (Jul 07, 2009) (7 pages) doi:10.1115/1.3156000 History: Received July 20, 2008; Revised April 22, 2009; Published July 07, 2009

An experimental investigation of the role of liquid transparency in controlling laser-induced motion of liquid drops is carried out. The study was motivated by application to manipulation of liquid drops over a solid substrate. Droplets with diameters of 1–4 mm were propelled on a hydrophobic substrate using a pulsed-laser beam (532 nm, 10 Hz, 3–12 mJ/pulse) with a 0.9 mm diameter fired parallel to the substrate. The test liquid was distilled water whose transparency was varied by adding different concentrations of Rhodamine 6G dye. Motion of the drops was observed using a video camera. Measurements include direction of motion and the distance traveled before the drops come to rest. The present results show that the direction of the motion depends on the drop transparency; opaque drops moved away from the laser beam, whereas transparent drops moved at small angles toward the laser beam. The motion of both transparent and opaque drops was dominated by thermal Marangoni effect; the motion of opaque drops was due to direct heating by the laser beam, whereas in the case of transparent drops, the laser beam was focused near the rear face of the transparent drops to form a spark that pushed the drops in the opposite direction. Energies lower than 3 mJ were incapable of moving the drops, and energies higher than 12 mJ shattered the drops instead of moving them. A phenomenological model was developed for the drop motion to explain the physics behind the phenomenon.

Copyright © 2009 by American Society of Mechanical Engineers
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Figure 1

(a) A transparent water drop on the Fluoropel® substrate with a contact angle of 169 deg and a drop diameter of 1.7 mm and (b) contact angle for a 0.2% Rhodamine 6G (by weight) water drop on Fluoropel® with a contact angle of 170 deg and a drop diameter of 1.6 mm

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Figure 2

Experimental setup. The video camera connected to the computer observes the top view of the substrate (to measure the direction of the motion), whereas the still camera observes the side view (to measure the contact angle).

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Figure 3

Laser-induced motion for (a) a transparent drop, (b) a translucent drop, and (c) an opaque drop. The long arrow on the photo represents the laser beam direction. The small arrow on the photo (and the dotted arrow in the sketch) indicates the direction of the motion of the drop in each case. The squares in the background of (a) are 5×5 mm2.

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Figure 4

Measurements of the direction of the movement and the distance traveled. The laser beam comes from the north.

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Figure 5

Schematic of the present model. The heated volume is divided into ten cells for the model.



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