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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 scattering profiles of hydrodynamically focusedflowing 5CB. The coordinates (qh, ql) and angle χ in reciprocal space aredefined in Figure 2a. (a) qh scans integrated over different ql intervals in the center of the channel (z ≈ 0 μm) for a hydrodynamic focusing experiment. Theflow rate of 5CB (middle channel) is Qm = 3 μL/min, and the ratio rQ = 2Qs/Qm of the combined side fluids (Triton X-100 2% solutions) with theinner fluid is rQ = 8.(b) Comparison between qh scans (integratedover ql = −0.036 to 0.036 Å–1) in the system with no hydrodynamic focusing (black, Q = 3 μL/min) and the hydrodynamic focusing system(Q5CB = 3 μL/min, rQ = 8). All hydrodynamic focusing patternsshow a slight shift in the peak position to lower angles (Δq ≈ 0.006 Å–1), which mayindicate a slight penetration/contamination of Triton or Triton/waterparticles within the 5CB sheet. Nonetheless, given the very smallshift in the peak, both the contamination and resulting structuralchanges should be minimal. (c) χ scans and respective double-Lorentzianfittings for a hydrodynamic focusing run with Qm = 3 μL/min and rQ = 8. Data is normalized and displaced along the ordinate axisfor ease of visualization. In all panels (a–c), circles constitutescattering data and lines are Lorentzian fits.
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fig7: One-dimensional scattering profiles of hydrodynamically focusedflowing 5CB. The coordinates (qh, ql) and angle χ in reciprocal space aredefined in Figure 2a. (a) qh scans integrated over different ql intervals in the center of the channel (z ≈ 0 μm) for a hydrodynamic focusing experiment. Theflow rate of 5CB (middle channel) is Qm = 3 μL/min, and the ratio rQ = 2Qs/Qm of the combined side fluids (Triton X-100 2% solutions) with theinner fluid is rQ = 8.(b) Comparison between qh scans (integratedover ql = −0.036 to 0.036 Å–1) in the system with no hydrodynamic focusing (black, Q = 3 μL/min) and the hydrodynamic focusing system(Q5CB = 3 μL/min, rQ = 8). All hydrodynamic focusing patternsshow a slight shift in the peak position to lower angles (Δq ≈ 0.006 Å–1), which mayindicate a slight penetration/contamination of Triton or Triton/waterparticles within the 5CB sheet. Nonetheless, given the very smallshift in the peak, both the contamination and resulting structuralchanges should be minimal. (c) χ scans and respective double-Lorentzianfittings for a hydrodynamic focusing run with Qm = 3 μL/min and rQ = 8. Data is normalized and displaced along the ordinate axisfor ease of visualization. In all panels (a–c), circles constitutescattering data and lines are Lorentzian fits.

Mentions: At the synchrotron,two flow conditions were studied: Qm =3 μL/min, rQ =8; and Qm = 1 μL/min, rQ = 24. In Figure 6e, 2D SAXS profiles due to scattering from the hydrodynamicallyfocused 5CB at different positions along the z axisare shown. As in the simple flow experiment, the same rotation inthe nematic director is observed, with θyz being negative in the upper half of 5CB and positive in thelower half. In this case, the beam size is much larger (fwhm ∼30μm), which introduces much stronger averaging effects but isstill suitable for an overall determination of the focused nematicstream. The larger scanning step size (10 and 20 μm) also needsto be taken into account. Figure 7a shows qh scans averaged over different ql intervals at Qm = 3 μL/min, rQ = 8, and z = 0 μm. In Figure 7b, qh scans of the central region of the pattern (ql interval of −0.036 to 0.036 Å–1) are shown across the nematic stream, in contactwith the Triton solutions, and compared to the pure system. All hydrodynamicfocusing patterns show a slight shift in the peak position to lowerangles (Δq ≈ 0.006 Å–1), which may indicate a slight penetration/contamination of tritonor triton/water particles within the 5CB sheet. Even though this shiftin the peak is very small, this phenomenon is occurring mainly onthe interface, which makes the signal weaker compared to that of theremaining nematic sheet. Hence, greater structural changes cannotbe discarded. We also note that although the residence time of 5CBin the channel is small (∼0.1 s), the surfactant concentrationis two orders of magnitude above the cmc (0.019 wt %37), which could lead to some solubilization of 5CB at theinterface and lead to some instabilities that could affect the observedresults.


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 scattering profiles of hydrodynamically focusedflowing 5CB. The coordinates (qh, ql) and angle χ in reciprocal space aredefined in Figure 2a. (a) qh scans integrated over different ql intervals in the center of the channel (z ≈ 0 μm) for a hydrodynamic focusing experiment. Theflow rate of 5CB (middle channel) is Qm = 3 μL/min, and the ratio rQ = 2Qs/Qm of the combined side fluids (Triton X-100 2% solutions) with theinner fluid is rQ = 8.(b) Comparison between qh scans (integratedover ql = −0.036 to 0.036 Å–1) in the system with no hydrodynamic focusing (black, Q = 3 μL/min) and the hydrodynamic focusing system(Q5CB = 3 μL/min, rQ = 8). All hydrodynamic focusing patternsshow a slight shift in the peak position to lower angles (Δq ≈ 0.006 Å–1), which mayindicate a slight penetration/contamination of Triton or Triton/waterparticles within the 5CB sheet. Nonetheless, given the very smallshift in the peak, both the contamination and resulting structuralchanges should be minimal. (c) χ scans and respective double-Lorentzianfittings for a hydrodynamic focusing run with Qm = 3 μL/min and rQ = 8. Data is normalized and displaced along the ordinate axisfor ease of visualization. In all panels (a–c), circles constitutescattering data and lines are Lorentzian fits.
© Copyright Policy
Related In: Results  -  Collection

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

fig7: One-dimensional scattering profiles of hydrodynamically focusedflowing 5CB. The coordinates (qh, ql) and angle χ in reciprocal space aredefined in Figure 2a. (a) qh scans integrated over different ql intervals in the center of the channel (z ≈ 0 μm) for a hydrodynamic focusing experiment. Theflow rate of 5CB (middle channel) is Qm = 3 μL/min, and the ratio rQ = 2Qs/Qm of the combined side fluids (Triton X-100 2% solutions) with theinner fluid is rQ = 8.(b) Comparison between qh scans (integratedover ql = −0.036 to 0.036 Å–1) in the system with no hydrodynamic focusing (black, Q = 3 μL/min) and the hydrodynamic focusing system(Q5CB = 3 μL/min, rQ = 8). All hydrodynamic focusing patternsshow a slight shift in the peak position to lower angles (Δq ≈ 0.006 Å–1), which mayindicate a slight penetration/contamination of Triton or Triton/waterparticles within the 5CB sheet. Nonetheless, given the very smallshift in the peak, both the contamination and resulting structuralchanges should be minimal. (c) χ scans and respective double-Lorentzianfittings for a hydrodynamic focusing run with Qm = 3 μL/min and rQ = 8. Data is normalized and displaced along the ordinate axisfor ease of visualization. In all panels (a–c), circles constitutescattering data and lines are Lorentzian fits.
Mentions: At the synchrotron,two flow conditions were studied: Qm =3 μL/min, rQ =8; and Qm = 1 μL/min, rQ = 24. In Figure 6e, 2D SAXS profiles due to scattering from the hydrodynamicallyfocused 5CB at different positions along the z axisare shown. As in the simple flow experiment, the same rotation inthe nematic director is observed, with θyz being negative in the upper half of 5CB and positive in thelower half. In this case, the beam size is much larger (fwhm ∼30μm), which introduces much stronger averaging effects but isstill suitable for an overall determination of the focused nematicstream. The larger scanning step size (10 and 20 μm) also needsto be taken into account. Figure 7a shows qh scans averaged over different ql intervals at Qm = 3 μL/min, rQ = 8, and z = 0 μm. In Figure 7b, qh scans of the central region of the pattern (ql interval of −0.036 to 0.036 Å–1) are shown across the nematic stream, in contactwith the Triton solutions, and compared to the pure system. All hydrodynamicfocusing patterns show a slight shift in the peak position to lowerangles (Δq ≈ 0.006 Å–1), which may indicate a slight penetration/contamination of tritonor triton/water particles within the 5CB sheet. Even though this shiftin the peak is very small, this phenomenon is occurring mainly onthe interface, which makes the signal weaker compared to that of theremaining nematic sheet. Hence, greater structural changes cannotbe discarded. We also note that although the residence time of 5CBin the channel is small (∼0.1 s), the surfactant concentrationis two orders of magnitude above the cmc (0.019 wt %37), which could lead to some solubilization of 5CB at theinterface and lead to some instabilities that could affect the observedresults.

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