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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 hydrodynamic focusing experimentswith the nematicphase of 5CB confined in a microfluidic device by solid and liquidinterfaces. (a) Schematic representation of the experiment. 5CB flowsalong channel 1 and is focused into channel 4 by the side fluids (2wt % Triton X-100 in water) coming from channels 2 and 3. This setupfacilitates the formation of stable nematic sheets. (b) Schematicrepresentation of the 5CB nematic sheet focused by the Triton solution.(c) Micrograph of a typical hydrodynamic focusing experiment. Herethe side fluids contain 0.1 mM fluorescein to facilitate the visualizationof the nematic sheet (black). 5CB flows at 3 μL/min, and eachside fluid flows at 12 μL/min (rQ = 2Qs/Qm = 8). The width of the nematic sheet is ca. 46 μmon the top and ca. 19 μm in the middle of the channel. The greenbox represents the X-ray beam’s fwhm (∼30 μm along z and ∼3000 μm along y) andhence the area probed by each X-ray measurement. The orange box representsa typical area probed in a vertical scan (z intervalsof 10–20 μm). (d) The same as in (c) but with a 5CB flowrate of 1 μL/min and rQ = 24. The focused nematic sheet seems to have a concave shapealong the x direction (note the relief pattern indicatedby the white arrow compared to the dark arrow for the formation ofthe sheet), with its width being ca. 22 μm on the top and ca.8 μm in the middle of the channel. (e) SAXS along the widthof the channel for Qm = 3 μL/min, rQ = 8. As in the simple flowexperiments, here also the nematic director rotates from a negativeθyz on the top to a positive θyz on the bottom side of the sheet.
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fig6: Summary of the hydrodynamic focusing experimentswith the nematicphase of 5CB confined in a microfluidic device by solid and liquidinterfaces. (a) Schematic representation of the experiment. 5CB flowsalong channel 1 and is focused into channel 4 by the side fluids (2wt % Triton X-100 in water) coming from channels 2 and 3. This setupfacilitates the formation of stable nematic sheets. (b) Schematicrepresentation of the 5CB nematic sheet focused by the Triton solution.(c) Micrograph of a typical hydrodynamic focusing experiment. Herethe side fluids contain 0.1 mM fluorescein to facilitate the visualizationof the nematic sheet (black). 5CB flows at 3 μL/min, and eachside fluid flows at 12 μL/min (rQ = 2Qs/Qm = 8). The width of the nematic sheet is ca. 46 μmon the top and ca. 19 μm in the middle of the channel. The greenbox represents the X-ray beam’s fwhm (∼30 μm along z and ∼3000 μm along y) andhence the area probed by each X-ray measurement. The orange box representsa typical area probed in a vertical scan (z intervalsof 10–20 μm). (d) The same as in (c) but with a 5CB flowrate of 1 μL/min and rQ = 24. The focused nematic sheet seems to have a concave shapealong the x direction (note the relief pattern indicatedby the white arrow compared to the dark arrow for the formation ofthe sheet), with its width being ca. 22 μm on the top and ca.8 μm in the middle of the channel. (e) SAXS along the widthof the channel for Qm = 3 μL/min, rQ = 8. As in the simple flowexperiments, here also the nematic director rotates from a negativeθyz on the top to a positive θyz on the bottom side of the sheet.

Mentions: In section 3.1, we described the alignmentof 5CB and the interplay between viscous and surface forces in simpleflow, where the boundary conditions are due to the walls of the microchannel(solid interfaces). In this section, we describe a similar approach,but where the nematic liquid crystal is bound by flowing liquid fromabove and below (liquid interfaces) and where capillary forces arisingfrom the interfacial tension between the immiscible fluids (nematicand water with some dissolved surfactant) also play an important role.For this purpose, we employ the technique of hydrodynamic focusing.20,34,44 In this experiment, 5CB flowsalong the main channel (channel 1, cf. Figure 6a,b) as before, with a flow rate of Qm, but now a solution of 2 wt % Triton X-100 also flows in channels2 and 3 at a flow rate of Qs (Figure 6a). The ratio of the total flow rate from the sidechannels (2Qs) to the flow rate of themiddle fluid (Qm) is rQ = 2Qs/Qm. The three fluids meet at the cross and flowtogether into the outlet (channel 4). For a suitable range of flowrates Qm and flow rate ratios rQ between 5CB and the tritonsolutions, stable sheets of 5CB with fluid on both sides can form(Table S2 and Figure 6c,d). The nematic sheet was found to be stable for a surfactant solutiontotal flow rate of 2Qs = 24 μL/min,with 5CB flowing in the range of Qm =1 to 3 μL/min (Table S2). At lower Qm < 1 μL/min, jetting occurs, witha thin thread of 5CB forming for a limited length (∼200–300μm) before rupturing to form droplets. At higher Qm > 3 μL/min, displacement instabilities44 occur, leading to an invasion of middle fluid5CB into the side channels. The system is also stable when increasing Qm while keeping the same flow rate ratio rQ, but the same does not happenwhen Qm is lowered at constant rQ. This is because the systembecomes more sensitive to small perturbations. The middle fluid 5CBis not surrounded by side fluid in all directions. Instead, it issurrounded by side fluid along the z direction, inwhich the y–x interfacesare now the side fluid, but along the x direction,it is still bound by the chip walls (y–z surfaces, Figure 6b). (Note inFigure 6d that in the nematic region thereis no trace of fluorescence. Furthermore, the relief pattern indicatedby the white arrow suggests a concave shape of the nematic sheet.)It was found that Qm = 3 μL/minwith rQ = 8 is the moststable flow condition. For lower Qm (<1.5 μL/min), the flow condition becomes history-dependent becausethe nematic stream is not able to form by itself but remains stableif it is formed at higher Qm and thendecreased slowly to the final value.


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 hydrodynamic focusing experimentswith the nematicphase of 5CB confined in a microfluidic device by solid and liquidinterfaces. (a) Schematic representation of the experiment. 5CB flowsalong channel 1 and is focused into channel 4 by the side fluids (2wt % Triton X-100 in water) coming from channels 2 and 3. This setupfacilitates the formation of stable nematic sheets. (b) Schematicrepresentation of the 5CB nematic sheet focused by the Triton solution.(c) Micrograph of a typical hydrodynamic focusing experiment. Herethe side fluids contain 0.1 mM fluorescein to facilitate the visualizationof the nematic sheet (black). 5CB flows at 3 μL/min, and eachside fluid flows at 12 μL/min (rQ = 2Qs/Qm = 8). The width of the nematic sheet is ca. 46 μmon the top and ca. 19 μm in the middle of the channel. The greenbox represents the X-ray beam’s fwhm (∼30 μm along z and ∼3000 μm along y) andhence the area probed by each X-ray measurement. The orange box representsa typical area probed in a vertical scan (z intervalsof 10–20 μm). (d) The same as in (c) but with a 5CB flowrate of 1 μL/min and rQ = 24. The focused nematic sheet seems to have a concave shapealong the x direction (note the relief pattern indicatedby the white arrow compared to the dark arrow for the formation ofthe sheet), with its width being ca. 22 μm on the top and ca.8 μm in the middle of the channel. (e) SAXS along the widthof the channel for Qm = 3 μL/min, rQ = 8. As in the simple flowexperiments, here also the nematic director rotates from a negativeθyz on the top to a positive θyz on the bottom side of the sheet.
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Related In: Results  -  Collection

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Show All Figures
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fig6: Summary of the hydrodynamic focusing experimentswith the nematicphase of 5CB confined in a microfluidic device by solid and liquidinterfaces. (a) Schematic representation of the experiment. 5CB flowsalong channel 1 and is focused into channel 4 by the side fluids (2wt % Triton X-100 in water) coming from channels 2 and 3. This setupfacilitates the formation of stable nematic sheets. (b) Schematicrepresentation of the 5CB nematic sheet focused by the Triton solution.(c) Micrograph of a typical hydrodynamic focusing experiment. Herethe side fluids contain 0.1 mM fluorescein to facilitate the visualizationof the nematic sheet (black). 5CB flows at 3 μL/min, and eachside fluid flows at 12 μL/min (rQ = 2Qs/Qm = 8). The width of the nematic sheet is ca. 46 μmon the top and ca. 19 μm in the middle of the channel. The greenbox represents the X-ray beam’s fwhm (∼30 μm along z and ∼3000 μm along y) andhence the area probed by each X-ray measurement. The orange box representsa typical area probed in a vertical scan (z intervalsof 10–20 μm). (d) The same as in (c) but with a 5CB flowrate of 1 μL/min and rQ = 24. The focused nematic sheet seems to have a concave shapealong the x direction (note the relief pattern indicatedby the white arrow compared to the dark arrow for the formation ofthe sheet), with its width being ca. 22 μm on the top and ca.8 μm in the middle of the channel. (e) SAXS along the widthof the channel for Qm = 3 μL/min, rQ = 8. As in the simple flowexperiments, here also the nematic director rotates from a negativeθyz on the top to a positive θyz on the bottom side of the sheet.
Mentions: In section 3.1, we described the alignmentof 5CB and the interplay between viscous and surface forces in simpleflow, where the boundary conditions are due to the walls of the microchannel(solid interfaces). In this section, we describe a similar approach,but where the nematic liquid crystal is bound by flowing liquid fromabove and below (liquid interfaces) and where capillary forces arisingfrom the interfacial tension between the immiscible fluids (nematicand water with some dissolved surfactant) also play an important role.For this purpose, we employ the technique of hydrodynamic focusing.20,34,44 In this experiment, 5CB flowsalong the main channel (channel 1, cf. Figure 6a,b) as before, with a flow rate of Qm, but now a solution of 2 wt % Triton X-100 also flows in channels2 and 3 at a flow rate of Qs (Figure 6a). The ratio of the total flow rate from the sidechannels (2Qs) to the flow rate of themiddle fluid (Qm) is rQ = 2Qs/Qm. The three fluids meet at the cross and flowtogether into the outlet (channel 4). For a suitable range of flowrates Qm and flow rate ratios rQ between 5CB and the tritonsolutions, stable sheets of 5CB with fluid on both sides can form(Table S2 and Figure 6c,d). The nematic sheet was found to be stable for a surfactant solutiontotal flow rate of 2Qs = 24 μL/min,with 5CB flowing in the range of Qm =1 to 3 μL/min (Table S2). At lower Qm < 1 μL/min, jetting occurs, witha thin thread of 5CB forming for a limited length (∼200–300μm) before rupturing to form droplets. At higher Qm > 3 μL/min, displacement instabilities44 occur, leading to an invasion of middle fluid5CB into the side channels. The system is also stable when increasing Qm while keeping the same flow rate ratio rQ, but the same does not happenwhen Qm is lowered at constant rQ. This is because the systembecomes more sensitive to small perturbations. The middle fluid 5CBis not surrounded by side fluid in all directions. Instead, it issurrounded by side fluid along the z direction, inwhich the y–x interfacesare now the side fluid, but along the x direction,it is still bound by the chip walls (y–z surfaces, Figure 6b). (Note inFigure 6d that in the nematic region thereis no trace of fluorescence. Furthermore, the relief pattern indicatedby the white arrow suggests a concave shape of the nematic sheet.)It was found that Qm = 3 μL/minwith rQ = 8 is the moststable flow condition. For lower Qm (<1.5 μL/min), the flow condition becomes history-dependent becausethe nematic stream is not able to form by itself but remains stableif it is formed at higher Qm and thendecreased slowly to the final value.

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