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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

Fitting results for the nematic directorangle projection θyz at different z positionsalong the microchannel (cf. Figure 1c for thedefinition of θyz). Results forthe (a) 0.2 μL/min ≤ Q ≤ 3 μL/minand (b) Q < 0.1 μL/min regimes. The z position is rescaled by centering θ = 0° at zrescaled = 0 μm. Note that the abscissarepresents /θyz/ in reverse orderfor ease of visualization because the resulting z – θyz dependenceresembles the Poiseuille flow profile. Solid data points result fromX-ray patterns where the beam is mostly inside the microchannel inthe y–z plane. Open datapoints result from patterns where a significant fraction of the X-raybeam was hitting outside of the y–z plane, catching part of the y–x plane because of a small rotation of the microfluidicdevice holder about the y axis parallel to the flow(i.e., the β angle shown in Figure 1a).Note that some points close to z = 55 μm shiftto smaller angles. A clear distinction in the flow profiles can beseen for Q ≥ 0.2 and Q <0.1 μL/min. In the first group (a), the director tilt is controlledby flow (i.e., the Leslie solution discussed in the text, where thenematic director assumes a dominant angle ±θS). The red and blue lines represent the Leslie solution for θS = 11° using the Poiseuille and non-Newtonian velocityprofiles, respectively (which are very similar). In the second group(b), boundary effects are more predominant. The line represents theescaped director configuration model (eq 3)assuming R = 71 μm (half of the diagonal lengthof the square channel), resulting in θR (the angle of n at the surface) = 32°.(c) Schematic representation of the orientation of the nematic directoras a function of microchannel z position for Q = 3 and ≈0 μL/min. Each blue bar representsthe average orientation of a rectangle with height Δz ≈ 6 μm and length Δy ≈ 40 μm (the fwhm of the beam profile). The directorangle θyz is multiplied by 3 tofacilitate visualization of the changes across z andfor the different Q values. (d) Schematic illustrationof the molecular arrangement for selected points along the orientationfield in (b) and respective X-ray scattering patterns.
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fig5: Fitting results for the nematic directorangle projection θyz at different z positionsalong the microchannel (cf. Figure 1c for thedefinition of θyz). Results forthe (a) 0.2 μL/min ≤ Q ≤ 3 μL/minand (b) Q < 0.1 μL/min regimes. The z position is rescaled by centering θ = 0° at zrescaled = 0 μm. Note that the abscissarepresents /θyz/ in reverse orderfor ease of visualization because the resulting z – θyz dependenceresembles the Poiseuille flow profile. Solid data points result fromX-ray patterns where the beam is mostly inside the microchannel inthe y–z plane. Open datapoints result from patterns where a significant fraction of the X-raybeam was hitting outside of the y–z plane, catching part of the y–x plane because of a small rotation of the microfluidicdevice holder about the y axis parallel to the flow(i.e., the β angle shown in Figure 1a).Note that some points close to z = 55 μm shiftto smaller angles. A clear distinction in the flow profiles can beseen for Q ≥ 0.2 and Q <0.1 μL/min. In the first group (a), the director tilt is controlledby flow (i.e., the Leslie solution discussed in the text, where thenematic director assumes a dominant angle ±θS). The red and blue lines represent the Leslie solution for θS = 11° using the Poiseuille and non-Newtonian velocityprofiles, respectively (which are very similar). In the second group(b), boundary effects are more predominant. The line represents theescaped director configuration model (eq 3)assuming R = 71 μm (half of the diagonal lengthof the square channel), resulting in θR (the angle of n at the surface) = 32°.(c) Schematic representation of the orientation of the nematic directoras a function of microchannel z position for Q = 3 and ≈0 μL/min. Each blue bar representsthe average orientation of a rectangle with height Δz ≈ 6 μm and length Δy ≈ 40 μm (the fwhm of the beam profile). The directorangle θyz is multiplied by 3 tofacilitate visualization of the changes across z andfor the different Q values. (d) Schematic illustrationof the molecular arrangement for selected points along the orientationfield in (b) and respective X-ray scattering patterns.

Mentions: In Figure 5a,b, we show the dependence ofθyz on z for flowrates of 0.2 μL/min ≤ Q ≤ 3 μL/minand Q < 0.1 μL/min. Note that to facilitatethe observation of symmetry (or lack thereof) of θyz across z with a mirror plane inthe middle of the channel (z = 0 μm), we displaythe absolute value /θyz/ instead.Also, for easier association of the θyz dependence with the Poiseuille profile, the abscissa θyz is displayed in reverse order. Immediatelyevident is the fact that for Q ≥ 0.2 μL/minall of the curves overlap very well. Likewise, Q approaching0 and Q ≈ 0 μL/min also display verysimilar profiles to each other. More striking is the obvious differencein curve profiles between the Q ≥ 0.2 and Q < 0.1 μL/min groups. In all cases, θyz is negative (clockwise rotation) in theupper half of the microchannel (z > 0) and ispositive(counterclockwise rotation) in the lower half (z <0). The same information can be seen in Figure 5c for flow rates of Q = 3 μL/min and Q ≈ 0 μL/min and where the nematic directorprojection nyz orientationis represented in the form of an orientation field as a function of z and y. Figure 5d shows a schematic of the 5CB molecules and nematic director orientationand corresponding 2D SAXS patterns.


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)

Fitting results for the nematic directorangle projection θyz at different z positionsalong the microchannel (cf. Figure 1c for thedefinition of θyz). Results forthe (a) 0.2 μL/min ≤ Q ≤ 3 μL/minand (b) Q < 0.1 μL/min regimes. The z position is rescaled by centering θ = 0° at zrescaled = 0 μm. Note that the abscissarepresents /θyz/ in reverse orderfor ease of visualization because the resulting z – θyz dependenceresembles the Poiseuille flow profile. Solid data points result fromX-ray patterns where the beam is mostly inside the microchannel inthe y–z plane. Open datapoints result from patterns where a significant fraction of the X-raybeam was hitting outside of the y–z plane, catching part of the y–x plane because of a small rotation of the microfluidicdevice holder about the y axis parallel to the flow(i.e., the β angle shown in Figure 1a).Note that some points close to z = 55 μm shiftto smaller angles. A clear distinction in the flow profiles can beseen for Q ≥ 0.2 and Q <0.1 μL/min. In the first group (a), the director tilt is controlledby flow (i.e., the Leslie solution discussed in the text, where thenematic director assumes a dominant angle ±θS). The red and blue lines represent the Leslie solution for θS = 11° using the Poiseuille and non-Newtonian velocityprofiles, respectively (which are very similar). In the second group(b), boundary effects are more predominant. The line represents theescaped director configuration model (eq 3)assuming R = 71 μm (half of the diagonal lengthof the square channel), resulting in θR (the angle of n at the surface) = 32°.(c) Schematic representation of the orientation of the nematic directoras a function of microchannel z position for Q = 3 and ≈0 μL/min. Each blue bar representsthe average orientation of a rectangle with height Δz ≈ 6 μm and length Δy ≈ 40 μm (the fwhm of the beam profile). The directorangle θyz is multiplied by 3 tofacilitate visualization of the changes across z andfor the different Q values. (d) Schematic illustrationof the molecular arrangement for selected points along the orientationfield in (b) and respective X-ray scattering patterns.
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Related In: Results  -  Collection

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

fig5: Fitting results for the nematic directorangle projection θyz at different z positionsalong the microchannel (cf. Figure 1c for thedefinition of θyz). Results forthe (a) 0.2 μL/min ≤ Q ≤ 3 μL/minand (b) Q < 0.1 μL/min regimes. The z position is rescaled by centering θ = 0° at zrescaled = 0 μm. Note that the abscissarepresents /θyz/ in reverse orderfor ease of visualization because the resulting z – θyz dependenceresembles the Poiseuille flow profile. Solid data points result fromX-ray patterns where the beam is mostly inside the microchannel inthe y–z plane. Open datapoints result from patterns where a significant fraction of the X-raybeam was hitting outside of the y–z plane, catching part of the y–x plane because of a small rotation of the microfluidicdevice holder about the y axis parallel to the flow(i.e., the β angle shown in Figure 1a).Note that some points close to z = 55 μm shiftto smaller angles. A clear distinction in the flow profiles can beseen for Q ≥ 0.2 and Q <0.1 μL/min. In the first group (a), the director tilt is controlledby flow (i.e., the Leslie solution discussed in the text, where thenematic director assumes a dominant angle ±θS). The red and blue lines represent the Leslie solution for θS = 11° using the Poiseuille and non-Newtonian velocityprofiles, respectively (which are very similar). In the second group(b), boundary effects are more predominant. The line represents theescaped director configuration model (eq 3)assuming R = 71 μm (half of the diagonal lengthof the square channel), resulting in θR (the angle of n at the surface) = 32°.(c) Schematic representation of the orientation of the nematic directoras a function of microchannel z position for Q = 3 and ≈0 μL/min. Each blue bar representsthe average orientation of a rectangle with height Δz ≈ 6 μm and length Δy ≈ 40 μm (the fwhm of the beam profile). The directorangle θyz is multiplied by 3 tofacilitate visualization of the changes across z andfor the different Q values. (d) Schematic illustrationof the molecular arrangement for selected points along the orientationfield in (b) and respective X-ray scattering patterns.
Mentions: In Figure 5a,b, we show the dependence ofθyz on z for flowrates of 0.2 μL/min ≤ Q ≤ 3 μL/minand Q < 0.1 μL/min. Note that to facilitatethe observation of symmetry (or lack thereof) of θyz across z with a mirror plane inthe middle of the channel (z = 0 μm), we displaythe absolute value /θyz/ instead.Also, for easier association of the θyz dependence with the Poiseuille profile, the abscissa θyz is displayed in reverse order. Immediatelyevident is the fact that for Q ≥ 0.2 μL/minall of the curves overlap very well. Likewise, Q approaching0 and Q ≈ 0 μL/min also display verysimilar profiles to each other. More striking is the obvious differencein curve profiles between the Q ≥ 0.2 and Q < 0.1 μL/min groups. In all cases, θyz is negative (clockwise rotation) in theupper half of the microchannel (z > 0) and ispositive(counterclockwise rotation) in the lower half (z <0). The same information can be seen in Figure 5c for flow rates of Q = 3 μL/min and Q ≈ 0 μL/min and where the nematic directorprojection nyz orientationis represented in the form of an orientation field as a function of z and y. Figure 5d shows a schematic of the 5CB molecules and nematic director orientationand corresponding 2D SAXS patterns.

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