We present birefringence and fluorescence confocal microscopy studies of melamine particles in a ... more We present birefringence and fluorescence confocal microscopy studies of melamine particles in a liquid-crystalline host solvent. The liquid crystal has a cholesteric phase at room temperature with a helical pitch that can be modified by changing the composition. The pitch employed here is always less than the particle diameter (3 microm). We demonstrate via birefringence that the mesogens preferentially anchor flat at the melamine surface. Via studies in a sample cell with aligning surfaces we show that although the particles form chains in a nematic liquid crystal they organize in 2D plates in a cholesteric. Fluorescence confocal microscopy and particle location analysis are used to determine the radial distribution function and to evaluate the particle aggregation number as a function of pitch length. We discuss possible explanations for the self-organization.
Proceedings of SPIE - The International Society for Optical Engineering
Many applications of laser tweezers rely on the accurate measurement of the transverse or axial t... more Many applications of laser tweezers rely on the accurate measurement of the transverse or axial trapping force. We have concentrated on the transverse trapping force and the most common method used to measure it, applying a viscous drag force. A trapped sphere was subjected to a viscous drag force via a Stokesian flow. The flow was achieved by oscillating the sample stage at a constant speed of 750 microns/second. A Zeiss oil-immersion (N.A. equals 1.3) objective was used to focus a 1064 nm Nd:YVO4 laser beam in order to trap 6 microns diameter polystyrene spheres suspended in distilled water. The minimum power needed to hold the particle in the trap at a particular viscous drag force was then measured. The influence of trap depth, oscillation amplitude used and particle concentration have been investigated, in particular the effects caused by the characteristics of the function used to create the oscillation. The minimum laser power needed to trap a sphere was found to increase with a rise in oscillation amplitude. The velocity profile through the fluid, the rotation of the trapped particle and the effect of interactions with other particles is considered when explaining these effects.
Page 1. Mechanisms of optical angular momentum transfer to nematic liquid crystalline droplets Ti... more Page 1. Mechanisms of optical angular momentum transfer to nematic liquid crystalline droplets Tiffany A. Wood, Helen F. Gleeson,a) Mark R. Dickinson, and Amanda J. Wright Department of Physics and Astronomy, University ...
Page 1. + Taylor & Francis JOURNAL OF MODERN OPTICS, 2003, VOL. 50, NO. 10, 1... more Page 1. + Taylor & Francis JOURNAL OF MODERN OPTICS, 2003, VOL. 50, NO. 10, 1521-1532 0 Taylor&Fmuir Group The transverse trapping force of an optical trap: factors affecting its measurement AMANDA J. WRIGHT, TIFFANY ...
ABSTRACT Whilst the behavior of particles trapped at liquid-liquid interfaces is relatively well ... more ABSTRACT Whilst the behavior of particles trapped at liquid-liquid interfaces is relatively well understood the behavior as one of the phases begins to break translational symmetry is almost completely unexplored. Here the particles seed defects in the partially ordered liquid and new, effective, particle-particle interactions are induced. We use a chiral (cholesteric) liquid crystal which has a characteristic length scale, the pitch length, similar to the particle size. Our system consists of particles with planar anchoring which are trapped at an interface between the liquid crystal and an isotropic liquid (silicone oil which induces homeotropic anchoring). The creation of the cholesteric ``fingerprint'' texture allows the deformation of the cholesteric around a particle to be easily visualized. This allows us to determine the nature of the defects created and their symmetries. We have clear trends for the distribution of particles with respect to the interface as a function of particle size. Inspired by computer simulations we study the position of small particles (diameter < pitch length) within the fingerprint texture. The behavior of the unadorned interface between the chiral liquid crystal and the oil is also explored.
We present birefringence and fluorescence confocal microscopy studies of melamine particles in a ... more We present birefringence and fluorescence confocal microscopy studies of melamine particles in a liquid-crystalline host solvent. The liquid crystal has a cholesteric phase at room temperature with a helical pitch that can be modified by changing the composition. The pitch employed here is always less than the particle diameter (3 microm). We demonstrate via birefringence that the mesogens preferentially anchor flat at the melamine surface. Via studies in a sample cell with aligning surfaces we show that although the particles form chains in a nematic liquid crystal they organize in 2D plates in a cholesteric. Fluorescence confocal microscopy and particle location analysis are used to determine the radial distribution function and to evaluate the particle aggregation number as a function of pitch length. We discuss possible explanations for the self-organization.
Proceedings of SPIE - The International Society for Optical Engineering
Many applications of laser tweezers rely on the accurate measurement of the transverse or axial t... more Many applications of laser tweezers rely on the accurate measurement of the transverse or axial trapping force. We have concentrated on the transverse trapping force and the most common method used to measure it, applying a viscous drag force. A trapped sphere was subjected to a viscous drag force via a Stokesian flow. The flow was achieved by oscillating the sample stage at a constant speed of 750 microns/second. A Zeiss oil-immersion (N.A. equals 1.3) objective was used to focus a 1064 nm Nd:YVO4 laser beam in order to trap 6 microns diameter polystyrene spheres suspended in distilled water. The minimum power needed to hold the particle in the trap at a particular viscous drag force was then measured. The influence of trap depth, oscillation amplitude used and particle concentration have been investigated, in particular the effects caused by the characteristics of the function used to create the oscillation. The minimum laser power needed to trap a sphere was found to increase with a rise in oscillation amplitude. The velocity profile through the fluid, the rotation of the trapped particle and the effect of interactions with other particles is considered when explaining these effects.
Page 1. Mechanisms of optical angular momentum transfer to nematic liquid crystalline droplets Ti... more Page 1. Mechanisms of optical angular momentum transfer to nematic liquid crystalline droplets Tiffany A. Wood, Helen F. Gleeson,a) Mark R. Dickinson, and Amanda J. Wright Department of Physics and Astronomy, University ...
Page 1. + Taylor & Francis JOURNAL OF MODERN OPTICS, 2003, VOL. 50, NO. 10, 1... more Page 1. + Taylor & Francis JOURNAL OF MODERN OPTICS, 2003, VOL. 50, NO. 10, 1521-1532 0 Taylor&Fmuir Group The transverse trapping force of an optical trap: factors affecting its measurement AMANDA J. WRIGHT, TIFFANY ...
ABSTRACT Whilst the behavior of particles trapped at liquid-liquid interfaces is relatively well ... more ABSTRACT Whilst the behavior of particles trapped at liquid-liquid interfaces is relatively well understood the behavior as one of the phases begins to break translational symmetry is almost completely unexplored. Here the particles seed defects in the partially ordered liquid and new, effective, particle-particle interactions are induced. We use a chiral (cholesteric) liquid crystal which has a characteristic length scale, the pitch length, similar to the particle size. Our system consists of particles with planar anchoring which are trapped at an interface between the liquid crystal and an isotropic liquid (silicone oil which induces homeotropic anchoring). The creation of the cholesteric ``fingerprint'' texture allows the deformation of the cholesteric around a particle to be easily visualized. This allows us to determine the nature of the defects created and their symmetries. We have clear trends for the distribution of particles with respect to the interface as a function of particle size. Inspired by computer simulations we study the position of small particles (diameter < pitch length) within the fingerprint texture. The behavior of the unadorned interface between the chiral liquid crystal and the oil is also explored.
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