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RESEARCH PAPERS: Non-Newtonian Behavior and Rheology

# Flow Induced Unstable Structure of Liquid Crystalline Polymer Solution in L-Shaped Slit Channels

[+] Author and Article Information
Takatsune Narumi

Faculty of Engineering, Niigata University, 2-8050 Ikarashi, Nishi Ward, Niigata 950-2181, Japannarumi@eng.niigata-u.ac.jp

Jun Fukada, Satoru Kiryu, Shinji Toga

Graduate School of Science and Technology, Niigata University, 2-8050 Ikarashi, Nishi Ward, Niigata 950-2181, Japan

Tomiichi Hasegawa

Faculty of Engineering, Niigata University, 2-8050 Ikarashi, Nishi Ward, Niigata 950-2181, Japan

J. Fluids Eng 130(8), 081503 (Jul 24, 2008) (6 pages) doi:10.1115/1.2956604 History: Received May 15, 2007; Revised August 28, 2007; Published July 24, 2008

## Abstract

An experimental study has been conducted on unstable structures induced in two-dimensional slit flows of liquid crystalline polymer solution. $50wt%$ aqueous solution of hydroxyl-propylcellulose (HPC) was utilized as a test fluid and its flow behavior in L-shaped slit channels with a cross section of $1mm$ height and $16mm$ width was measured optically. The inner corner of the L-shaped channel was rounded off in order to clarify the influence of the radius of curvature on the unstable behavior. A conversing curved channel was also tested. The flow patterns of the HPC solution in the channels were visualized with two crossed polarizers and we observed that typical wavy textures generated in the upstream of the corner almost disappeared after the corner flow. However, an unstable texture was developed again only from the inner corner in downstream flow. The fluctuation of the orientation angle and dichroism were also measured with a laser opto-rheometric system and it was found that the unstable behaviors of the HPC solution have periodic oscillatory characteristics at a typical frequency. In the inner side flow after the corner, the periodic motion became larger toward the downstream and then higher harmonic oscillations were superimposed. Larger rounding off of the inner corner suppressed the redevelopment of unstable behavior, and it is considered that the rapid regrowth of unstable behavior was caused by rapid deceleration at the corner flow. Moreover, the unstable structure was stabilized with an accelerated (elongated) region in the corner flow and the converging channel was helpful to obtain a stable structure in the downstream region.

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## Figures

Figure 1

Schema of the test channel

Figure 2

Shape of a flow cell

Figure 3

Experimental apparatus

Figure 4

Measurement positions of the orientation angle and dichroism

Figure 5

Photographs of flow patterns observed in the (a) ST channel and the (b) R2 channel between two crossed polaraizers with orientations of (45deg, −45deg) to the flow direction (a) ST channel γ̇w=7.6(1∕s) (b) R2 channel γ̇w=7.7(1∕s)

Figure 6

Orientation angle measured at the inner side of the downstream of the R2 channel at γ̇w=7.9(1∕s)

Figure 7

Power spectrum of the fluctuation in the orientation angle obtained with digital Fourier transformation from Fig. 6 (R2 channel)

Figure 8

Instability of the downstream flow evaluated as the rms of the fluctuation in orientation angle measured with the R2 channel at γ̇w=7.9(1∕s)

Figure 9

Power spectrum of the fluctuation in orientation angle obtained with digital Fourier transformation (R10 channel, γ̇w=7.8(1∕s))

Figure 10

Instability of the downstream flow evaluated as the rms of the fluctuation in orientation angle (R10 channel, γ̇w=7.8(1∕s))

Figure 11

Instability change around the corner to the downstream region (a) R2 channel at γ̇w=7.9(1∕s) (b) R10 channel at γ̇w=7.8(1∕s)

Figure 12

Velocity vectors in the corner flow obtained with bubbles’ motion in the HPC solution (a) R2 channel at γ̇w=7.5(1∕s) (b) R10 channel at γ̇w=7.3(1∕s)

Figure 13

Local velocity changes on pass lines in the corner flows. Inner pass lines are about 2mm away from the inside edge. The outer lines past through the o-1 point and the pass length was measured from the −1 position in Fig. 4. γ̇w=7.5(1∕s) in the R2 channel flow and γ̇w=7.3(1∕s) in the R10 channel flow.

Figure 14

Photographs of flow patterns observed in the CONV channel between two crossed polarizers with orientations of (45deg, −45deg) to the flow direction. Shear rate (γ̇w) changes from 7.4to14.8(1∕s).

Figure 15

Instability of the downstream flow evaluated as the rms of the fluctuation in orientation angle (CONV channel, γ̇w=14.8(1∕s) in the downstream region)

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