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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

Results for the nematic director angle projection θyz at different z positionsalongthe microchannel in the hydrodynamic focusing experiments. (a) Comparisonof measured θyz for the simple flowexperiments (no hydrodynamic focusing) at Q = 3 μL/minusing the SSRL (green) and cSAXS (blue, finer beam size) beamlines.Even with the loss in spatial resolution due to the ca. 5-fold-largerX-ray beam at SSRL, which also shifts θyz to lower values, the same overall evolution in director rotationis observed. (b) Comparison between the simple flow and hydrodynamicfocusing systems. The symbols represent measured θyz, and the lines are guides for the eye. The z position is rescaled by centering the director angle θ= 0° at zrescaled = 0 μm.
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fig8: Results for the nematic director angle projection θyz at different z positionsalongthe microchannel in the hydrodynamic focusing experiments. (a) Comparisonof measured θyz for the simple flowexperiments (no hydrodynamic focusing) at Q = 3 μL/minusing the SSRL (green) and cSAXS (blue, finer beam size) beamlines.Even with the loss in spatial resolution due to the ca. 5-fold-largerX-ray beam at SSRL, which also shifts θyz to lower values, the same overall evolution in director rotationis observed. (b) Comparison between the simple flow and hydrodynamicfocusing systems. The symbols represent measured θyz, and the lines are guides for the eye. The z position is rescaled by centering the director angle θ= 0° at zrescaled = 0 μm.

Mentions: As with the simple flow case (cf. section 3.1), also here it is convenient to extract θyz from the χ scans. Figure 7c showsthe resulting fits of scans with the double-Lorentzian expression(eq 2). Figure 8 showsa plot of the measured θyz at different z positions along the channel. The data extends over a wider z range than the widths of the nematic sheets wm. This results from the fact that the beam size (fwhm≈ 30 μm) is larger than the nematic stream, which enlargesthe range in z over which the nematic phase givesa signal. More importantly, the averaging effects due to beam sizealso modify the observed values of θyz. In Figure 8a, a comparison of thesimple flow system (i.e., no hydrodynamic focusing) with the small(fwhm ≈ 6 μm, blue circles) and large (fwhm ≈30 μm, green squares) beams can be seen. The curves are slightlydifferent, with the data from the larger beam showing a shift to smallerθyz angles. Despite this, the sameoverall trend is still observed, which allows the characterizationof the system. The shift in θyz tosmaller angles is easily understood if one recalls that θyz has opposite signs in the upper and lowerhalves of the microchannel. If the beam is large enough that it partiallyoverlaps both halves, then the opposite signs of θyz partially cancel each other, making the observedangle θyz smaller. This effect ismore significant for narrower 5CB streams because the θyz variations are more compressed in space,accentuating the smearing from the larger beam size. In Figure 8b, /θyz/ versus z curves are shown for different rQ. Even with the finite beam size effects,clear differences in the slopes of the different curves are easilyobserved. It can be clearly seen that as rQ increases from rQ = 0 (simple flow experiments, green squares) to rQ = 8 (blue circles) and to rQ = 24 (red diamonds), the variationof /θyz/ with z also increases.


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)

Results for the nematic director angle projection θyz at different z positionsalongthe microchannel in the hydrodynamic focusing experiments. (a) Comparisonof measured θyz for the simple flowexperiments (no hydrodynamic focusing) at Q = 3 μL/minusing the SSRL (green) and cSAXS (blue, finer beam size) beamlines.Even with the loss in spatial resolution due to the ca. 5-fold-largerX-ray beam at SSRL, which also shifts θyz to lower values, the same overall evolution in director rotationis observed. (b) Comparison between the simple flow and hydrodynamicfocusing systems. The symbols represent measured θyz, and the lines are guides for the eye. The z position is rescaled by centering the director angle θ= 0° at zrescaled = 0 μm.
© Copyright Policy
Related In: Results  -  Collection

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

fig8: Results for the nematic director angle projection θyz at different z positionsalongthe microchannel in the hydrodynamic focusing experiments. (a) Comparisonof measured θyz for the simple flowexperiments (no hydrodynamic focusing) at Q = 3 μL/minusing the SSRL (green) and cSAXS (blue, finer beam size) beamlines.Even with the loss in spatial resolution due to the ca. 5-fold-largerX-ray beam at SSRL, which also shifts θyz to lower values, the same overall evolution in director rotationis observed. (b) Comparison between the simple flow and hydrodynamicfocusing systems. The symbols represent measured θyz, and the lines are guides for the eye. The z position is rescaled by centering the director angle θ= 0° at zrescaled = 0 μm.
Mentions: As with the simple flow case (cf. section 3.1), also here it is convenient to extract θyz from the χ scans. Figure 7c showsthe resulting fits of scans with the double-Lorentzian expression(eq 2). Figure 8 showsa plot of the measured θyz at different z positions along the channel. The data extends over a wider z range than the widths of the nematic sheets wm. This results from the fact that the beam size (fwhm≈ 30 μm) is larger than the nematic stream, which enlargesthe range in z over which the nematic phase givesa signal. More importantly, the averaging effects due to beam sizealso modify the observed values of θyz. In Figure 8a, a comparison of thesimple flow system (i.e., no hydrodynamic focusing) with the small(fwhm ≈ 6 μm, blue circles) and large (fwhm ≈30 μm, green squares) beams can be seen. The curves are slightlydifferent, with the data from the larger beam showing a shift to smallerθyz angles. Despite this, the sameoverall trend is still observed, which allows the characterizationof the system. The shift in θyz tosmaller angles is easily understood if one recalls that θyz has opposite signs in the upper and lowerhalves of the microchannel. If the beam is large enough that it partiallyoverlaps both halves, then the opposite signs of θyz partially cancel each other, making the observedangle θyz smaller. This effect ismore significant for narrower 5CB streams because the θyz variations are more compressed in space,accentuating the smearing from the larger beam size. In Figure 8b, /θyz/ versus z curves are shown for different rQ. Even with the finite beam size effects,clear differences in the slopes of the different curves are easilyobserved. It can be clearly seen that as rQ increases from rQ = 0 (simple flow experiments, green squares) to rQ = 8 (blue circles) and to rQ = 24 (red diamonds), the variationof /θyz/ with z also increases.

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