Limits...
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

Illustration of the chip design, microchannel geometry, and coordinatesystem. (a) The flow is along the +y direction (inlet1 to outlets 2–4). (b) The channel cross section is squarewith a width of 100 μm. The velocity profile can be describedwith some level of approximation by a Poiseuille flow being maximizedat the center of the channel and fading to zero at the surfaces asa result of the no-slip boundary condition. (c) Far from the edges,the nematic director n aligns preferentially alongthe velocity direction (y), making an angle θin the velocity gradient direction (along x and z). ϕ is the angle between the velocity gradient andthe z direction and defines the nematic directorcone around the velocity direction y. θyz is the angle between y and n projected onto the y–z plane (nyz). θyx is the angle between y (velocity direction) and n projectedonto the y–x plane (nyx). (d) Alternative visualizationof θyz, the angle measured in the2D SAXS detector (y–z plane),and θyx.
© Copyright Policy
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4400036&req=5

fig1: Illustration of the chip design, microchannel geometry, and coordinatesystem. (a) The flow is along the +y direction (inlet1 to outlets 2–4). (b) The channel cross section is squarewith a width of 100 μm. The velocity profile can be describedwith some level of approximation by a Poiseuille flow being maximizedat the center of the channel and fading to zero at the surfaces asa result of the no-slip boundary condition. (c) Far from the edges,the nematic director n aligns preferentially alongthe velocity direction (y), making an angle θin the velocity gradient direction (along x and z). ϕ is the angle between the velocity gradient andthe z direction and defines the nematic directorcone around the velocity direction y. θyz is the angle between y and n projected onto the y–z plane (nyz). θyx is the angle between y (velocity direction) and n projectedonto the y–x plane (nyx). (d) Alternative visualizationof θyz, the angle measured in the2D SAXS detector (y–z plane),and θyx.

Mentions: 5CB (4-cyano-4′-pentylbiphenyl) was purchased from KingstonChemicals. Triton X-100 (poly(ethylene glycol) p-(1,1,3,3-tetramethylbutyl)-phenylether) was purchased from Sigma. Fluorescein was purchased from Aldrich.All of these chemicals were used as received. In all experiments,high-purity Millipore water was used. The microfluidic chips, madeof cyclic olefin copolymer (COC or Topas) were purchased from MicrofluidicChipShop (catalog no. 02-0757-0166-02) and used without modificationof the hydrophobic surface. The channel consists of four inlets meetingat a cross, with inlets 1–4 having lengths of 80, 5, 5, and6 mm, respectively (Figure 1a). The microchannelcross section is square, with a width of 100 μm. In this work,contrary to what is more conventional, the main fluid flows in thereverse mode from what is usually the outlet along the main channel,meeting the cross at the end (Figure 1a). Thismethod allows the study of pure 5CB in simple flow (without hydrodynamicfocusing) along the main channel without perturbations from the cross.More importantly, in the hydrodynamic focusing experiments, allowingthe nematic to flow in this direction helps to stabilize the nematicsheet and avoid the displacement of the LC into side channels 2 and3.


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)

Illustration of the chip design, microchannel geometry, and coordinatesystem. (a) The flow is along the +y direction (inlet1 to outlets 2–4). (b) The channel cross section is squarewith a width of 100 μm. The velocity profile can be describedwith some level of approximation by a Poiseuille flow being maximizedat the center of the channel and fading to zero at the surfaces asa result of the no-slip boundary condition. (c) Far from the edges,the nematic director n aligns preferentially alongthe velocity direction (y), making an angle θin the velocity gradient direction (along x and z). ϕ is the angle between the velocity gradient andthe z direction and defines the nematic directorcone around the velocity direction y. θyz is the angle between y and n projected onto the y–z plane (nyz). θyx is the angle between y (velocity direction) and n projectedonto the y–x plane (nyx). (d) Alternative visualizationof θyz, the angle measured in the2D SAXS detector (y–z plane),and θyx.
© Copyright Policy
Related In: Results  -  Collection

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

fig1: Illustration of the chip design, microchannel geometry, and coordinatesystem. (a) The flow is along the +y direction (inlet1 to outlets 2–4). (b) The channel cross section is squarewith a width of 100 μm. The velocity profile can be describedwith some level of approximation by a Poiseuille flow being maximizedat the center of the channel and fading to zero at the surfaces asa result of the no-slip boundary condition. (c) Far from the edges,the nematic director n aligns preferentially alongthe velocity direction (y), making an angle θin the velocity gradient direction (along x and z). ϕ is the angle between the velocity gradient andthe z direction and defines the nematic directorcone around the velocity direction y. θyz is the angle between y and n projected onto the y–z plane (nyz). θyx is the angle between y (velocity direction) and n projectedonto the y–x plane (nyx). (d) Alternative visualizationof θyz, the angle measured in the2D SAXS detector (y–z plane),and θyx.
Mentions: 5CB (4-cyano-4′-pentylbiphenyl) was purchased from KingstonChemicals. Triton X-100 (poly(ethylene glycol) p-(1,1,3,3-tetramethylbutyl)-phenylether) was purchased from Sigma. Fluorescein was purchased from Aldrich.All of these chemicals were used as received. In all experiments,high-purity Millipore water was used. The microfluidic chips, madeof cyclic olefin copolymer (COC or Topas) were purchased from MicrofluidicChipShop (catalog no. 02-0757-0166-02) and used without modificationof the hydrophobic surface. The channel consists of four inlets meetingat a cross, with inlets 1–4 having lengths of 80, 5, 5, and6 mm, respectively (Figure 1a). The microchannelcross section is square, with a width of 100 μm. In this work,contrary to what is more conventional, the main fluid flows in thereverse mode from what is usually the outlet along the main channel,meeting the cross at the end (Figure 1a). Thismethod allows the study of pure 5CB in simple flow (without hydrodynamicfocusing) along the main channel without perturbations from the cross.More importantly, in the hydrodynamic focusing experiments, allowingthe nematic to flow in this direction helps to stabilize the nematicsheet and avoid the displacement of the LC into side channels 2 and3.

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