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Zhou X, Zhang Z. Dynamics of order reconstruction in a nanoconfined nematic liquid crystal with a topological defect. Int J Mol Sci 2013; 14:24135-53. [PMID: 24351807 PMCID: PMC3876101 DOI: 10.3390/ijms141224135] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/26/2013] [Revised: 11/26/2013] [Accepted: 11/27/2013] [Indexed: 11/16/2022] Open
Abstract
At the wall in a hybrid nematic cell with strong anchoring, the nematic director is parallel to one wall and perpendicular to the other. Within the Landau-de Gennes theory, we have investigated the dynamics of s = ±1/2 wedge disclinations in such a cell, using the two-dimensional finite-difference iterative method. Our results show that with the cell gap decreasing, the core of the defect explodes, and the biaxiality propagates inside the cell. At a critical value of dc* ≈ 9ξ (where ξ is the characteristic length for order-parameter changes), the exchange solution is stable, while the defect core solution becomes metastable. Comparing to the case with no initial disclination, the value at which the exchange solution becomes stable increases relatively. At a critical separation of dc ≈ 6ξ, the system undergoes a structural transition, and the defect core merges into a biaxial layer with large biaxiality. For weak anchoring boundary conditions, a similar structural transition takes place at a relative lower critical value. Because of the weakened frustration, the asymmetric boundary conditions repel the defect to the weak anchoring boundary and have a relatively lower critical value of da, where the shape of the defect deforms. Further, the response time between two very close cell gaps is about tens of microseconds, and the response becomes slower as the defect explodes.
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Affiliation(s)
- Xuan Zhou
- Department of physics, Hebei University of Technology, Tianjin 300401, China; E-Mail:
| | - Zhidong Zhang
- Department of physics, Hebei University of Technology, Tianjin 300401, China; E-Mail:
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Bobrovsky A, Sinitsyna O, Abramchuk S, Yaminsky I, Shibaev V. Atomic force microscopy study of surface topography of films of cholesteric oligomer- and polymer-based mixtures with photovariable helix pitch. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2013; 87:012503. [PMID: 23410345 DOI: 10.1103/physreve.87.012503] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/14/2012] [Indexed: 06/01/2023]
Abstract
The surface topography of glass-forming polymers and oligomer cholesteric systems with a phototunable helix pitch was studied. For this purpose several mixtures based on nematic polyacrylate and cholesteric cyclosiloxanes doped with chiral-photochromic dopant were prepared and investigated. The molecules of chiral-photochromic dopant consist of isosorbide chiral moiety and cinnamic C=C double bonds capable of E-Z photoisomerizing. UV irradiation of the planarly oriented films of mixtures leads to dopant photoisomerization and changes of its helical twisting power. During this process irreversible changes of helix pitch values take place, which allows one to obtain the same cholesteric systems with different helix pitch values. The films of the annealed mixtures were studied by atomic force microscopy and transmission electron microscopy. The correlations between the features of surface topography and helix pitch of cholesteric supramolecular structure were found and discussed.
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Affiliation(s)
- Alexey Bobrovsky
- Faculty of Chemistry, Moscow State University, Lenin Hills, Moscow, 119991 Russia.
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Leite FL, Bueno CC, Da Róz AL, Ziemath EC, Oliveira ON. Theoretical models for surface forces and adhesion and their measurement using atomic force microscopy. Int J Mol Sci 2012. [PMID: 23202925 PMCID: PMC3497299 DOI: 10.3390/ijms131012773] [Citation(s) in RCA: 142] [Impact Index Per Article: 11.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022] Open
Abstract
The increasing importance of studies on soft matter and their impact on new technologies, including those associated with nanotechnology, has brought intermolecular and surface forces to the forefront of physics and materials science, for these are the prevailing forces in micro and nanosystems. With experimental methods such as the atomic force spectroscopy (AFS), it is now possible to measure these forces accurately, in addition to providing information on local material properties such as elasticity, hardness and adhesion. This review provides the theoretical and experimental background of afs, adhesion forces, intermolecular interactions and surface forces in air, vacuum and in solution.
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Affiliation(s)
- Fabio L. Leite
- Nanoneurobiophysics Research Group, Department of Physics, Chemistry and Mathematics, Federal University of São Carlos (UFSCar), P.O. Box 3031, CEP 18052-780, Sorocaba, São Paulo, Brazil; E-Mails: (C.C.B.); (A.L.D.R.)
- Author to whom correspondence should be addressed; E-Mail: ; Tel.: +55-015-3229-6014; Fax: +55-015-3229-5902
| | - Carolina C. Bueno
- Nanoneurobiophysics Research Group, Department of Physics, Chemistry and Mathematics, Federal University of São Carlos (UFSCar), P.O. Box 3031, CEP 18052-780, Sorocaba, São Paulo, Brazil; E-Mails: (C.C.B.); (A.L.D.R.)
| | - Alessandra L. Da Róz
- Nanoneurobiophysics Research Group, Department of Physics, Chemistry and Mathematics, Federal University of São Carlos (UFSCar), P.O. Box 3031, CEP 18052-780, Sorocaba, São Paulo, Brazil; E-Mails: (C.C.B.); (A.L.D.R.)
| | - Ervino C. Ziemath
- Institute of Geosciences and Exact Sciences, São Paulo State University (UNESP), P.O. Box 178, CEP 13550-970, Rio Claro, São Paulo, Brazil; E-Mail:
| | - Osvaldo N. Oliveira
- Institute of Physics of São Carlos, University of São Paulo (USP), P.O. Box 369, CEP 13560-970, São Carlos, São Paulo, Brazil; E-Mail:
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Carbone G, Zappone B, Barberi R, Bartolino R, Musevic I. Direct nanomechanical measurement of layer thickness and compressibility of smectic liquid crystals. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2011; 83:051707. [PMID: 21728556 DOI: 10.1103/physreve.83.051707] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/28/2010] [Revised: 04/12/2011] [Indexed: 05/31/2023]
Abstract
Using an atomic force microscope (AFM) we confined a smectic-A liquid crystal (LC) between a flat glass plate and a 10-μm glass sphere attached to the free end of the AFM cantilever. Both surfaces were treated with a surfactant that induces normal alignment of the LC molecules. We measured the force F acting on the cantilever while varying the plate-sphere distance D with subnanometer precision. For D < 50 nm, the force was periodically oscillating and decayed as D was increased. Analyzing the force in the framework of a simple model of elastic deformation of the smectic layers, we have evaluated the undeformed layer thickness a(0) and compressibility modulus B. Compared to other techniques used to determine a(0) and B, AFM measurements are faster and require a much smaller amount (microliters) of LC. Moreover, they are based on purely mechanical deformations of the LC structure and do not require any static or radiative electromagnetic field.
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Carbone G, Barberi R, Musevic I, Krzic U. Atomic force microscope study of presmectic modulation in the nematic and isotropic phases of the liquid crystal octylcyanobiphenyl using piezoresistive force detection. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2005; 71:051704. [PMID: 16089548 DOI: 10.1103/physreve.71.051704] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/28/2005] [Indexed: 05/03/2023]
Abstract
Using a temperature controlled atomic force microscope (AFM), we have studied surface induced pre-smectic order in the nematic and isotropic phases of 4-cyano- 4'-n -octylbiphenyl. A modified AFM head with piezoresitive cantilevers has been used to measure the structural force between a flat BK7 glass plate and a 10 microm glass sphere, both being treated to induce homeotropic alignment of the confined liquid crystal layer in between. We have observed surface-induced presmectic force not only in the isotropic, but also in the nematic phase. We have measured the temperature dependencies of the presmectic force, the smectic correlation length xi and the smectic order parameter psi at the surface. The correlation length xi(T) shows a power-law temperature dependence with a critical exponent of nu=0.67 +/- 0.03 and the bare correlation length of xi(0) = (0.39 +/- 0.08) nm, in good agreement with x-ray data. The smectic density at the surface is psi(2)(S) =0.4 in the nematic phase and decreases in the isotropic phase.
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Affiliation(s)
- G Carbone
- Licryl - Liquid Crystal Laboratory, INFM Research Unit of Calabria c/o Physics Department, University of Calabria, 87036 Rende (CS), Italy
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Atomic force microscopy in structured liquids: remark on the interpretation of jumps in force curves. Colloids Surf A Physicochem Eng Asp 2005. [DOI: 10.1016/j.colsurfa.2004.10.065] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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Bisi F, Virga EG, Durand GE. Nanomechanics of order reconstruction in nematic liquid crystals. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2004; 70:042701. [PMID: 15600442 DOI: 10.1103/physreve.70.042701] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/16/2004] [Indexed: 05/24/2023]
Abstract
We employ a continuum model to compute both torque and force transmitted through a thin twist cell filled with a nematic liquid crystal and bounded by flat plates with anchorings at right angles. The transmitted torque vanishes at the order reconstruction threshold when the cell thickness is comparable with the biaxial coherence length. At the same point, the force diagram exhibits an angular point which disappears above a critical twist mismatch. Both torque and force diagrams against the cell's thickness fail to be monotonic when the total twist is near pi/2 .
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Affiliation(s)
- Fulvio Bisi
- Dipartimento di Matematica, Istituto Nazionale di Fisica della Materia, Universitá di Pavia, via Ferrata 1, 27100 Pavia, Italy
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Abstract
The structure of the fragile liquid-crystalline phases has a strong impact on the forces between bodies immersed in a liquid crystal (LC). We have equipped an atomic force microscope with a precise temperature control and measured various liquid-crystalline structural forces at temperatures close to the phase transitions. The observed forces agree well with predictions of Landau--de Gennes phenomenological theory of LCs, even at a nanoscale length. In addition to this, we have observed a molecular layer, adsorbed on the surfactant-covered glass surface, and determined its thickness and elastic properties.
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Affiliation(s)
- Klemen Kocevar
- J. Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
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Sun G, Bonaccurso E, Franz V, Butt HJ. Confined liquid: Simultaneous observation of a molecularly layered structure and hydrodynamic slip. J Chem Phys 2002. [DOI: 10.1063/1.1515970] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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Butt HJ, Franz V. Rupture of molecular thin films observed in atomic force microscopy. I. Theory. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2002; 66:031601. [PMID: 12366124 DOI: 10.1103/physreve.66.031601] [Citation(s) in RCA: 79] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/19/2002] [Indexed: 05/23/2023]
Abstract
In atomic force microscope studies of molecular thin films, a defined jump of the tip through the film is often observed once a certain threshold force has been exceeded. Here, we present a theory to describe this film rupture and to relate microscopic parameters to measurable quantities. We assume that the tip has to overcome an activation energy before the film ruptures. A universal relation between the force dependence of the activation energy and the approaching velocity of the tip is derived. Two complementary models for calculating the activation energy are presented: a continuum nucleation model and a discrete molecular model. Both models predict a narrow distribution of yield forces in agreement with experimental results.
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Affiliation(s)
- Hans-Jürgen Butt
- Max-Planck-Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany.
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Kocevar K, Musevic I. Surface-induced nematic and smectic order at a liquid-crystal-silanated-glass interface observed by atomic force spectroscopy and Brewster angle ellipsometry. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2002; 65:021703. [PMID: 11863541 DOI: 10.1103/physreve.65.021703] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/13/2001] [Indexed: 05/23/2023]
Abstract
We have investigated nematic and smectic surface-induced ordering in the isotropic phase of the 4-cyano-4'-n-octylbiphenyl liquid crystal on silanated BK7 and LaSF glass using atomic force spectroscopy and Brewster angle ellipsometry. We have observed complete wetting of the silanated glass surfaces with the nematic phase when approaching the isotropic-nematic phase transition from above. Using the atomic force spectroscopy at the same interfaces, we have detected a significant presmectic ordering that is enhanced at the nematic-isotropic transition. We have observed the first, strongly adsorbed layer of liquid-crystalline molecules underneath this presmectic ordering. This first molecular layer is laterally inhomogeneous with typically 100 nm voids and covers approximately 70% of the surface. It is stable far above the clearing point and is responsible for the surface-memory effect. The results have been analyzed using Landau-de Gennes theory. The surface coupling energies of the nematic and smectic order have been determined, as well as the coupling energy between the nematic and smectic order.
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Affiliation(s)
- K Kocevar
- J. Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
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Kitzerow HS. News about Soft Condensed Matter—A Liquid Crystal Meeting Review. Chemphyschem 2001; 2:628-32. [DOI: 10.1002/1439-7641(20011015)2:10<628::aid-cphc628>3.0.co;2-7] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
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13
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Kocevar K, Borstnik A, Musevic I, Zumer S. Capillary condensation of a nematic liquid crystal observed by force spectroscopy. PHYSICAL REVIEW LETTERS 2001; 86:5914-5917. [PMID: 11415392 DOI: 10.1103/physrevlett.86.5914] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/26/2001] [Indexed: 05/23/2023]
Abstract
We have observed capillary condensation in nanometer-thin films of nematic liquid crystals using force spectroscopy. The liquid crystal was confined to a submicron gap between a flat substrate and a microsphere, attached to the cantilever of an atomic force microscope. A long-ranged and strongly attractive force due to capillary condensation of a nematic phase was observed at temperatures close to the bulk isotropic-nematic phase transition. The critical point, terminating the first-order confined isotropic-nematic transition line, was determined for the first time.
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Affiliation(s)
- K Kocevar
- J. Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
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