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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

Summary of the microfluidicSAXS experiments. (a) Typical SAXSpattern (top) and schematic of the microfluidic device (bottom). Thedevice holder (shown at the bottom) can be translated in the x, y, and z directionsand rotated in β (about y) and γ (about x). The X-ray beam is along the x direction,and the SAXS pattern (in the qy, qz plane) iscollected in a detector behind the device. In both the chip and detector,the flow is along the +y direction. A typical SAXSpattern (flow rate Q = 3 μL/min, z = 30 μm) at small q is dominated by the nematicphase correlation peak, which is inversely proportional to the nearest-neighbordistance between the nematogens along their main axis. Therefore,the peak maximum q = 2πn/d is along the nematic director n projectedonto the y–z plane (nyz), and θyz is the angle between nyz and the flow direction (y). Thedirection qh is defined as being parallelto nyz, and ql is perpendicular to nyz. In our convention, χ = 0° is the negative qz axis, and the positive rotation is counterclockwise.(b) Snapshot of 5CB flowing slowly (approaching 0 μL/min) beforedata acquisition at the cSAXS beamline. Nematic Schlieren-like flowingtextures can be seen even without polarized light. The green box representsthe X-ray beam’s fwhm (∼6 μm along z and ∼40 μm along y) and hence thearea probed by each X-ray measurement. The orange box represents atypical area probed in a vertical scan (z intervalsof 4.85 μm). (c) Representative scattering patterns of a flowingnematic phase recorded along the width (z direction)of the microchannel at a flow rate of 3 μL/min (left) and atrest (right). The patterns rotate with roughly mirror symmetry withrespect to the center of the channel (clockwise in the upper partof the channel (+z) and counterclockwise in the lowerhalf (−z)). Close to the edges, the magnitudeof the rotation is larger at rest. At the edges of the microchannel,the scattering intensity is also lower because approximately halfof the X-ray beam is outside of the microchannel and may have a contributionfrom the y–x plane due toa small tilt (<2°) of the device in β.
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fig2: Summary of the microfluidicSAXS experiments. (a) Typical SAXSpattern (top) and schematic of the microfluidic device (bottom). Thedevice holder (shown at the bottom) can be translated in the x, y, and z directionsand rotated in β (about y) and γ (about x). The X-ray beam is along the x direction,and the SAXS pattern (in the qy, qz plane) iscollected in a detector behind the device. In both the chip and detector,the flow is along the +y direction. A typical SAXSpattern (flow rate Q = 3 μL/min, z = 30 μm) at small q is dominated by the nematicphase correlation peak, which is inversely proportional to the nearest-neighbordistance between the nematogens along their main axis. Therefore,the peak maximum q = 2πn/d is along the nematic director n projectedonto the y–z plane (nyz), and θyz is the angle between nyz and the flow direction (y). Thedirection qh is defined as being parallelto nyz, and ql is perpendicular to nyz. In our convention, χ = 0° is the negative qz axis, and the positive rotation is counterclockwise.(b) Snapshot of 5CB flowing slowly (approaching 0 μL/min) beforedata acquisition at the cSAXS beamline. Nematic Schlieren-like flowingtextures can be seen even without polarized light. The green box representsthe X-ray beam’s fwhm (∼6 μm along z and ∼40 μm along y) and hence thearea probed by each X-ray measurement. The orange box represents atypical area probed in a vertical scan (z intervalsof 4.85 μm). (c) Representative scattering patterns of a flowingnematic phase recorded along the width (z direction)of the microchannel at a flow rate of 3 μL/min (left) and atrest (right). The patterns rotate with roughly mirror symmetry withrespect to the center of the channel (clockwise in the upper partof the channel (+z) and counterclockwise in the lowerhalf (−z)). Close to the edges, the magnitudeof the rotation is larger at rest. At the edges of the microchannel,the scattering intensity is also lower because approximately halfof the X-ray beam is outside of the microchannel and may have a contributionfrom the y–x plane due toa small tilt (<2°) of the device in β.

Mentions: After the initial flow mapping throughmicroscopy and the identificationof stable flow regions, selected flow regimes are probed by SAXS.For the transmission SAXS measurements, the chip is mounted on a custom-builtdevice holder, with translation capacity in the x, y, and z directions, and rotationof the β (about y) and γ (about x) angles (Figure 2a, bottom). Beforethe measurements, γ is aligned to zero with high precision.The β alignment is more complicated, and a maximum tilt angleof ±2° can occur. The sample is placed normal to the X-raybeam and scanned in the z and y directions.The measurements were performed at two synchrotron facilities. Thesimple flow experiments were performed on the cSAXS beamline at theSwiss Light Source (SLS), Paul Scherrer Institute (PSI), Villigen,Switzerland. The hydrodynamic focusing experiments were performedon SAXS/D beamline 4-2 at the Stanford Synchrotron Light Source (SSRL),Menlo Park, CA, USA. For the cSAXS instrument, the X-ray wavelength(λ) used was 1.11 Å–1, and the sample-to-detectordistance was 2.16 m. The data was collected on a Pilatus 2 M detector.The beamline optics allowed a very small beam size of fwhm ≈6 μm in the z direction and 40 μm inthe y direction. For the SAXS/D (BL 4-2) instrument,the X-ray wavelength (λ) used was 1.03 Å–1, and the sample-to-detector distance was 1.1 m. Data was collectedon a MX-225 Rayonix CCD detector. The X-ray beam had a size of ∼30μm (fwhm) in the z direction and 300 μmin the y direction. For both instruments, the obtained2D scattering patterns were corrected for background by subtractingthe 2D pattern of the chip outside of the microchannel corrected forthe different thicknesses. The resulting 2D background-corrected patternsare subsequently analyzed with Matlab routines to extract structuralinformation through qh and ql scans or the nematic director orientation through χscans (Figure 2a, top).


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)

Summary of the microfluidicSAXS experiments. (a) Typical SAXSpattern (top) and schematic of the microfluidic device (bottom). Thedevice holder (shown at the bottom) can be translated in the x, y, and z directionsand rotated in β (about y) and γ (about x). The X-ray beam is along the x direction,and the SAXS pattern (in the qy, qz plane) iscollected in a detector behind the device. In both the chip and detector,the flow is along the +y direction. A typical SAXSpattern (flow rate Q = 3 μL/min, z = 30 μm) at small q is dominated by the nematicphase correlation peak, which is inversely proportional to the nearest-neighbordistance between the nematogens along their main axis. Therefore,the peak maximum q = 2πn/d is along the nematic director n projectedonto the y–z plane (nyz), and θyz is the angle between nyz and the flow direction (y). Thedirection qh is defined as being parallelto nyz, and ql is perpendicular to nyz. In our convention, χ = 0° is the negative qz axis, and the positive rotation is counterclockwise.(b) Snapshot of 5CB flowing slowly (approaching 0 μL/min) beforedata acquisition at the cSAXS beamline. Nematic Schlieren-like flowingtextures can be seen even without polarized light. The green box representsthe X-ray beam’s fwhm (∼6 μm along z and ∼40 μm along y) and hence thearea probed by each X-ray measurement. The orange box represents atypical area probed in a vertical scan (z intervalsof 4.85 μm). (c) Representative scattering patterns of a flowingnematic phase recorded along the width (z direction)of the microchannel at a flow rate of 3 μL/min (left) and atrest (right). The patterns rotate with roughly mirror symmetry withrespect to the center of the channel (clockwise in the upper partof the channel (+z) and counterclockwise in the lowerhalf (−z)). Close to the edges, the magnitudeof the rotation is larger at rest. At the edges of the microchannel,the scattering intensity is also lower because approximately halfof the X-ray beam is outside of the microchannel and may have a contributionfrom the y–x plane due toa small tilt (<2°) of the device in β.
© Copyright Policy
Related In: Results  -  Collection

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

fig2: Summary of the microfluidicSAXS experiments. (a) Typical SAXSpattern (top) and schematic of the microfluidic device (bottom). Thedevice holder (shown at the bottom) can be translated in the x, y, and z directionsand rotated in β (about y) and γ (about x). The X-ray beam is along the x direction,and the SAXS pattern (in the qy, qz plane) iscollected in a detector behind the device. In both the chip and detector,the flow is along the +y direction. A typical SAXSpattern (flow rate Q = 3 μL/min, z = 30 μm) at small q is dominated by the nematicphase correlation peak, which is inversely proportional to the nearest-neighbordistance between the nematogens along their main axis. Therefore,the peak maximum q = 2πn/d is along the nematic director n projectedonto the y–z plane (nyz), and θyz is the angle between nyz and the flow direction (y). Thedirection qh is defined as being parallelto nyz, and ql is perpendicular to nyz. In our convention, χ = 0° is the negative qz axis, and the positive rotation is counterclockwise.(b) Snapshot of 5CB flowing slowly (approaching 0 μL/min) beforedata acquisition at the cSAXS beamline. Nematic Schlieren-like flowingtextures can be seen even without polarized light. The green box representsthe X-ray beam’s fwhm (∼6 μm along z and ∼40 μm along y) and hence thearea probed by each X-ray measurement. The orange box represents atypical area probed in a vertical scan (z intervalsof 4.85 μm). (c) Representative scattering patterns of a flowingnematic phase recorded along the width (z direction)of the microchannel at a flow rate of 3 μL/min (left) and atrest (right). The patterns rotate with roughly mirror symmetry withrespect to the center of the channel (clockwise in the upper partof the channel (+z) and counterclockwise in the lowerhalf (−z)). Close to the edges, the magnitudeof the rotation is larger at rest. At the edges of the microchannel,the scattering intensity is also lower because approximately halfof the X-ray beam is outside of the microchannel and may have a contributionfrom the y–x plane due toa small tilt (<2°) of the device in β.
Mentions: After the initial flow mapping throughmicroscopy and the identificationof stable flow regions, selected flow regimes are probed by SAXS.For the transmission SAXS measurements, the chip is mounted on a custom-builtdevice holder, with translation capacity in the x, y, and z directions, and rotationof the β (about y) and γ (about x) angles (Figure 2a, bottom). Beforethe measurements, γ is aligned to zero with high precision.The β alignment is more complicated, and a maximum tilt angleof ±2° can occur. The sample is placed normal to the X-raybeam and scanned in the z and y directions.The measurements were performed at two synchrotron facilities. Thesimple flow experiments were performed on the cSAXS beamline at theSwiss Light Source (SLS), Paul Scherrer Institute (PSI), Villigen,Switzerland. The hydrodynamic focusing experiments were performedon SAXS/D beamline 4-2 at the Stanford Synchrotron Light Source (SSRL),Menlo Park, CA, USA. For the cSAXS instrument, the X-ray wavelength(λ) used was 1.11 Å–1, and the sample-to-detectordistance was 2.16 m. The data was collected on a Pilatus 2 M detector.The beamline optics allowed a very small beam size of fwhm ≈6 μm in the z direction and 40 μm inthe y direction. For the SAXS/D (BL 4-2) instrument,the X-ray wavelength (λ) used was 1.03 Å–1, and the sample-to-detector distance was 1.1 m. Data was collectedon a MX-225 Rayonix CCD detector. The X-ray beam had a size of ∼30μm (fwhm) in the z direction and 300 μmin the y direction. For both instruments, the obtained2D scattering patterns were corrected for background by subtractingthe 2D pattern of the chip outside of the microchannel corrected forthe different thicknesses. The resulting 2D background-corrected patternsare subsequently analyzed with Matlab routines to extract structuralinformation through qh and ql scans or the nematic director orientation through χscans (Figure 2a, top).

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