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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

χ scans and respective double-Lorentzian fittings(cf. Figure 2a, top, for the definition ofangle χ in the(qh, ql) plane)measuring the orientational width of the nematic correlation peakfor 5CB in a microfluidic device at Q ≈ 0μL/min. Data is normalized and displaced along the ordinateaxis for ease of visualization. The quality of the data (and hencethe fitting) is reduced at the edges of the microfluidic device (z ≈ ±53 μm), as a result of part of theX-ray beam being outside the microchannel and hence not hitting the5CB nematic.
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fig4: χ scans and respective double-Lorentzian fittings(cf. Figure 2a, top, for the definition ofangle χ in the(qh, ql) plane)measuring the orientational width of the nematic correlation peakfor 5CB in a microfluidic device at Q ≈ 0μL/min. Data is normalized and displaced along the ordinateaxis for ease of visualization. The quality of the data (and hencethe fitting) is reduced at the edges of the microfluidic device (z ≈ ±53 μm), as a result of part of theX-ray beam being outside the microchannel and hence not hitting the5CB nematic.

Mentions: In Figure 4, normalized χ scans andrespective fits are shown for Q ≈ 0 μL/min.As can be seen, when going from the top to the bottom of the channel(bottom to top in Figure 4a), χ0 shifts from ca. 61° at z = 53 μm toca. 124° at z = −53 μm. In between,χ0 ≈ 90° at z = 0 μm.Because the flow direction is along the y axis andis at an angle of χ = 90°, the director angle with theflow direction θyz is directly extractedthrough θyz = χ0 – 90°.


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)

χ scans and respective double-Lorentzian fittings(cf. Figure 2a, top, for the definition ofangle χ in the(qh, ql) plane)measuring the orientational width of the nematic correlation peakfor 5CB in a microfluidic device at Q ≈ 0μL/min. Data is normalized and displaced along the ordinateaxis for ease of visualization. The quality of the data (and hencethe fitting) is reduced at the edges of the microfluidic device (z ≈ ±53 μm), as a result of part of theX-ray beam being outside the microchannel and hence not hitting the5CB nematic.
© Copyright Policy
Related In: Results  -  Collection

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

fig4: χ scans and respective double-Lorentzian fittings(cf. Figure 2a, top, for the definition ofangle χ in the(qh, ql) plane)measuring the orientational width of the nematic correlation peakfor 5CB in a microfluidic device at Q ≈ 0μL/min. Data is normalized and displaced along the ordinateaxis for ease of visualization. The quality of the data (and hencethe fitting) is reduced at the edges of the microfluidic device (z ≈ ±53 μm), as a result of part of theX-ray beam being outside the microchannel and hence not hitting the5CB nematic.
Mentions: In Figure 4, normalized χ scans andrespective fits are shown for Q ≈ 0 μL/min.As can be seen, when going from the top to the bottom of the channel(bottom to top in Figure 4a), χ0 shifts from ca. 61° at z = 53 μm toca. 124° at z = −53 μm. In between,χ0 ≈ 90° at z = 0 μm.Because the flow direction is along the y axis andis at an angle of χ = 90°, the director angle with theflow direction θyz is directly extractedthrough θyz = χ0 – 90°.

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