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Nematic director reorientation at solid and liquid interfaces under flow: SAXS studies in a microfluidic device.

Silva BF, Zepeda-Rosales M, Venkateswaran N, Fletcher BJ, Carter LG, Matsui T, Weiss TM, Han J, Li Y, Olsson U, Safinya CR - Langmuir (2014)

Bottom Line: At moderate-to-high flow rates, the nematic director is predominantly aligned in the flow direction, but with a small tilt angle of ∼±11° in the velocity gradient direction.The director tilt angle is constant throughout most of the channel width but switches sign when crossing the center of the channel, in agreement with the Ericksen-Leslie-Parodi (ELP) theory.The technique presented here could be applied to perform high-throughput measurements for assessing the influence of different surfactants on the orientation of nematic phases and may lead to further improvements in areas such as boundary lubrication and clarifying the nature of defect structures in LC displays.

View Article: PubMed Central - PubMed

Affiliation: §Division of Physical Chemistry, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden.

ABSTRACT
In this work we investigate the interplay between flow and boundary condition effects on the orientation field of a thermotropic nematic liquid crystal under flow and confinement in a microfluidic device. Two types of experiments were performed using synchrotron small-angle X-ray-scattering (SAXS). In the first, a nematic liquid crystal flows through a square-channel cross section at varying flow rates, while the nematic director orientation projected onto the velocity/velocity gradient plane is measured using a 2D detector. At moderate-to-high flow rates, the nematic director is predominantly aligned in the flow direction, but with a small tilt angle of ∼±11° in the velocity gradient direction. The director tilt angle is constant throughout most of the channel width but switches sign when crossing the center of the channel, in agreement with the Ericksen-Leslie-Parodi (ELP) theory. At low flow rates, boundary conditions begin to dominate, and a flow profile resembling the escaped radial director configuration is observed, where the director is seen to vary more smoothly from the edges (with homeotropic alignment) to the center of the channel. In the second experiment, hydrodynamic focusing is employed to confine the nematic phase into a sheet of liquid sandwiched between two layers of Triton X-100 aqueous solutions. The average nematic director orientation shifts to some extent from the flow direction toward the liquid boundaries, although it remains unclear if one tilt angle is dominant through most of the nematic sheet (with abrupt jumps near the boundaries) or if the tilt angle varies smoothly between two extreme values (∼90 and 0°). The technique presented here could be applied to perform high-throughput measurements for assessing the influence of different surfactants on the orientation of nematic phases and may lead to further improvements in areas such as boundary lubrication and clarifying the nature of defect structures in LC displays.

No MeSH data available.


Related in: MedlinePlus

One-dimensional scatteringprofiles of the 5CB nematic confinedin the microfluidic device. The coordinates (qh and ql) in reciprocal space aredefined in Figure 2a (top). (a) qh scans integrated over different ql intervals in the center of the channel (z ≈ 0 μm) at a flow rate of Q = 3 μL/min.(b) qh scans (integrated over a ql range from −0.036 to 0.036 Å–1) at Q = 3 μL/min and at different z positions along the channel (cf. Figure 2c, which shows that the z range goes from∼−50 to ∼+50 μm). The proximity to themicrochannel boundaries does not modify or distort the 1D profilesintegrated over ql. (c) qh scans (integrated over the ql range from −0.036 to 0.036 Å–1) atdifferent flow rates. The 1D profiles are not modified or distortedin the investigated flow rates. In both (b) and (c), the curves arenormalized and slightly offset for ease of visualization. In all panels(a–c), the circles constitute scattering data and the linesconstitute Lorentzian fits.
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fig3: One-dimensional scatteringprofiles of the 5CB nematic confinedin the microfluidic device. The coordinates (qh and ql) in reciprocal space aredefined in Figure 2a (top). (a) qh scans integrated over different ql intervals in the center of the channel (z ≈ 0 μm) at a flow rate of Q = 3 μL/min.(b) qh scans (integrated over a ql range from −0.036 to 0.036 Å–1) at Q = 3 μL/min and at different z positions along the channel (cf. Figure 2c, which shows that the z range goes from∼−50 to ∼+50 μm). The proximity to themicrochannel boundaries does not modify or distort the 1D profilesintegrated over ql. (c) qh scans (integrated over the ql range from −0.036 to 0.036 Å–1) atdifferent flow rates. The 1D profiles are not modified or distortedin the investigated flow rates. In both (b) and (c), the curves arenormalized and slightly offset for ease of visualization. In all panels(a–c), the circles constitute scattering data and the linesconstitute Lorentzian fits.

Mentions: Figure 3a shows qh scans through the peak maximum (qh =0.254 Å–1) of 5CB at Q = 3μL/min for the 2D SAXS patterns at z = 0 μm(Figure 2c) averaged over different ql intervals. Real-space dimension d = 2π/q = 24.7 Å is ca. 1.4 times thesize of a 5CB molecule, in good agreement with the suggestion that5CB molecules are predominantly in a dimeric form, with their benzenerings forming a pair, and the alkyl chains protruding outward.38,39 The half-width at half-maximum (hwhm) of the peak is 0.057 Å–1. This corresponds to a correlation domain with ∼1/0.057Å–1 = 17.5 Å, which is expected for liquidswith near-neighbor short-range positional order.


Nematic director reorientation at solid and liquid interfaces under flow: SAXS studies in a microfluidic device.

Silva BF, Zepeda-Rosales M, Venkateswaran N, Fletcher BJ, Carter LG, Matsui T, Weiss TM, Han J, Li Y, Olsson U, Safinya CR - Langmuir (2014)

One-dimensional scatteringprofiles of the 5CB nematic confinedin the microfluidic device. The coordinates (qh and ql) in reciprocal space aredefined in Figure 2a (top). (a) qh scans integrated over different ql intervals in the center of the channel (z ≈ 0 μm) at a flow rate of Q = 3 μL/min.(b) qh scans (integrated over a ql range from −0.036 to 0.036 Å–1) at Q = 3 μL/min and at different z positions along the channel (cf. Figure 2c, which shows that the z range goes from∼−50 to ∼+50 μm). The proximity to themicrochannel boundaries does not modify or distort the 1D profilesintegrated over ql. (c) qh scans (integrated over the ql range from −0.036 to 0.036 Å–1) atdifferent flow rates. The 1D profiles are not modified or distortedin the investigated flow rates. In both (b) and (c), the curves arenormalized and slightly offset for ease of visualization. In all panels(a–c), the circles constitute scattering data and the linesconstitute Lorentzian fits.
© Copyright Policy
Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC4400036&req=5

fig3: One-dimensional scatteringprofiles of the 5CB nematic confinedin the microfluidic device. The coordinates (qh and ql) in reciprocal space aredefined in Figure 2a (top). (a) qh scans integrated over different ql intervals in the center of the channel (z ≈ 0 μm) at a flow rate of Q = 3 μL/min.(b) qh scans (integrated over a ql range from −0.036 to 0.036 Å–1) at Q = 3 μL/min and at different z positions along the channel (cf. Figure 2c, which shows that the z range goes from∼−50 to ∼+50 μm). The proximity to themicrochannel boundaries does not modify or distort the 1D profilesintegrated over ql. (c) qh scans (integrated over the ql range from −0.036 to 0.036 Å–1) atdifferent flow rates. The 1D profiles are not modified or distortedin the investigated flow rates. In both (b) and (c), the curves arenormalized and slightly offset for ease of visualization. In all panels(a–c), the circles constitute scattering data and the linesconstitute Lorentzian fits.
Mentions: Figure 3a shows qh scans through the peak maximum (qh =0.254 Å–1) of 5CB at Q = 3μL/min for the 2D SAXS patterns at z = 0 μm(Figure 2c) averaged over different ql intervals. Real-space dimension d = 2π/q = 24.7 Å is ca. 1.4 times thesize of a 5CB molecule, in good agreement with the suggestion that5CB molecules are predominantly in a dimeric form, with their benzenerings forming a pair, and the alkyl chains protruding outward.38,39 The half-width at half-maximum (hwhm) of the peak is 0.057 Å–1. This corresponds to a correlation domain with ∼1/0.057Å–1 = 17.5 Å, which is expected for liquidswith near-neighbor short-range positional order.

Bottom Line: At moderate-to-high flow rates, the nematic director is predominantly aligned in the flow direction, but with a small tilt angle of ∼±11° in the velocity gradient direction.The director tilt angle is constant throughout most of the channel width but switches sign when crossing the center of the channel, in agreement with the Ericksen-Leslie-Parodi (ELP) theory.The technique presented here could be applied to perform high-throughput measurements for assessing the influence of different surfactants on the orientation of nematic phases and may lead to further improvements in areas such as boundary lubrication and clarifying the nature of defect structures in LC displays.

View Article: PubMed Central - PubMed

Affiliation: §Division of Physical Chemistry, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden.

ABSTRACT
In this work we investigate the interplay between flow and boundary condition effects on the orientation field of a thermotropic nematic liquid crystal under flow and confinement in a microfluidic device. Two types of experiments were performed using synchrotron small-angle X-ray-scattering (SAXS). In the first, a nematic liquid crystal flows through a square-channel cross section at varying flow rates, while the nematic director orientation projected onto the velocity/velocity gradient plane is measured using a 2D detector. At moderate-to-high flow rates, the nematic director is predominantly aligned in the flow direction, but with a small tilt angle of ∼±11° in the velocity gradient direction. The director tilt angle is constant throughout most of the channel width but switches sign when crossing the center of the channel, in agreement with the Ericksen-Leslie-Parodi (ELP) theory. At low flow rates, boundary conditions begin to dominate, and a flow profile resembling the escaped radial director configuration is observed, where the director is seen to vary more smoothly from the edges (with homeotropic alignment) to the center of the channel. In the second experiment, hydrodynamic focusing is employed to confine the nematic phase into a sheet of liquid sandwiched between two layers of Triton X-100 aqueous solutions. The average nematic director orientation shifts to some extent from the flow direction toward the liquid boundaries, although it remains unclear if one tilt angle is dominant through most of the nematic sheet (with abrupt jumps near the boundaries) or if the tilt angle varies smoothly between two extreme values (∼90 and 0°). The technique presented here could be applied to perform high-throughput measurements for assessing the influence of different surfactants on the orientation of nematic phases and may lead to further improvements in areas such as boundary lubrication and clarifying the nature of defect structures in LC displays.

No MeSH data available.


Related in: MedlinePlus