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.

1.
Watanabe
,
H.
,
Narumi
,
T.
,
Mikuma
,
R.
,
Kayaba
,
R.
, and
Hasegawa
,
T.
, 2006, “
Study of Structural Rearrangement of Liquid Crystalline Polymer Solution in a Transient Couette Flow With a Flow Direction Change (in Japanese)
,”
Nihon Reoroji Gakkaishi
0387-1533,
34
(
1
), pp.
9
15
.
2.
Guido
,
S.
,
Frallicciardi
,
P.
,
Grizzuti
,
N.
, and
Marrucci
,
G.
, 1994, “
Rheo-Optics of Hydroxypropylcellulose Solutions in Poiseuille Flow
,”
Rheol. Acta
0035-4511,
33
, pp.
22
28
.
3.
Baleo
,
J.-N.
, and
Navard
,
P.
, 1994, “
Rheo-Optics of Liquid-Crystalline Polymers in Complex Geometries
,”
J. Rheol.
0148-6055,
38
, pp.
1641
1655
.
4.
Bedford
,
B. B.
, and
Burghart
,
W. R.
, 1994, “
Molecular Orientation and Instability in Plane Poiseuille Flow of a Liquid-Crystalline Polymer
,”
J. Rheol.
0148-6055,
38
, pp.
1657
1679
.
5.
Bedford
,
B. B.
, and
Burghardt
,
W. R.
, 1996, “
Molecular Orientation of a Liquid-Crystalline Polymer Solution in Mixed Shear-Extensional Flows
,”
J. Rheol.
0148-6055,
40
, pp.
235
257
.
6.
Bedford
,
B. B.
,
,
D. K.
, Jr.
, and
Burghardt
,
W. R.
, 1997, “
Unstable Slit Flow of a Liquid-Crystalline Polymer Solution
,”
Rheol. Acta
0035-4511,
36
, pp.
384
396
.
7.
Haw
,
M. D.
, and
Navard
,
P.
, 2000, “
Divergent Channel Flow of a Liquid Crystalline Polymer Solution
,”
Rheol. Acta
0035-4511,
39
, pp.
280
293
.
8.
Mori
,
N.
,
Shindo
,
M.
,
Takahashi
,
T.
, and
Nakamura
,
K.
, 2001, “
Unstable Slit Flows of Liquid Crystalline Solution of Hydroxypropylcellulose
Nihon Reoroji Gakkaishi
0387-1533,
29
, pp.
89
94
.
9.
Mori
,
N.
,
Shindo
,
M.
,
Nakamura
,
K.
, and
Yamamoto
,
T.
, 2003, “
Pressure-Driven Startup Flows of Liquid Crystalline Polymers Through Slit Cells
,”
Rheol. Acta
0035-4511,
42
(
6
), pp.
527
533
.
10.
Mori
,
N.
,
Origi
,
S.
,
Numata
,
A.
, and
Yamamoto
,
T.
, 2005, “
Wavy Texture in Pressure-Driven Startup Flows of Liquid Crystalline Polymer Solutions Through a Slit Cell: Effects of Polymer Concentration and Solution Temperature
,”
Rheol. Acta
0035-4511,
44
(
5
), pp.
478
484
.
11.
Kiryu
,
S.
, 2006, “
Study on Flow Induced Structure of Liquid Crystalline Polymer Solution in Two-Dimensional Channels With a Flow Direction Change (in Japanese)
,” MS thesis, Niigata University, Niigata, Japan.