Daniele Finotello

AC calorimetric studies of phase transitions in porous substrates. Superfluid helium and liquid crystals

Abstract In this review, we detail the application of an AC calorimetry technique as it has been employed and optimized for the study of phase transitions, particularly the superfluid-to-normal transition and those undergone by liquid crystals systems while confined to geometries more restrictive than bulk. The theory of operation, the sample preparation, and the electronic equipment involved in such measurements are described in detail. Examples of its use in heat capacity measurements of two-dimensional helium films at sub-Kelvin temperatures and in liquid crystal studies, a discussion on the simultaneously measured phase shift, which contains information regarding the order of the phase transition, is also included.

Interface influenced phase transitions of alkylcyanobiphenyl liquid crystals A calorimetric study

Abstract Using an AC-calorimetry technique, we report measurements of the heat capacity and the phase shift between the applied heating power and the resulting thermal oscillations on the thermotropic liquid crystal series of alkylcyanobiphenyl (nCB) above the tricritical point. Specifically, we studied the first order phase transition smectic A to isotropic as a function of substrate, under atmospheric pressure. Different combinations of sapphire, Kapton type 200H polyimide film and air, are used to vary systematically the interfaces encountered by a liquid crystal. The calorimeter uses a sapphire disk, 10 mm in diameter and 0·13 mm thick, above which the samples are placed. A second sapphire disk, or a disk-shaped Kapton film, are used to sandwich the liquid crystal. Air interface samples are droplets allowed to spread naturally on the chosen substrate. The chosen geometry is such that interface effects appear to be maximized. Striking features found in the heat capacity and the phase shift studies with...

Liquid Crystal Order in a Highly Restrictive Porous Glass

Abstract The thermodynamic and structural properties of octylcyanobiphenyl (8CB) confined to the highly restrictive and randomly interconnected pores of Vycor glass were studied via AC calorimetry, DSC, and small angle neutron scattering. The weakly first order nematic to isotropic phase transition is absent and replaced by a continuous evolution of local orientational ordering with decreasing temperature. The results are consistent with predictions from a model that treats the host media as a collection of noninteracting pore segments. The continuous smectic-A to nematic phase transition is also absent with no evidence of any smectic ordering. In addition, crystallization is replaced by a glass-like melting transition with indications of another melting transition at much lower temperatures.

HIGH RESOLUTION CALORIMETRIC STUDIES AT PHASE TRANSITIONS OF ALKYLCYANOBIPHENYL LIQUID CRYSTALS CONFINED TO SUBMICRON SIZE CYLINDRICAL CAVITIES

We review results of a high resolution systematic study of the specific heat for alkyl-cyanobiphenyl liquid crystals confined to the 0.2µm diameter cylindrical pores Anopore membranes. The nematic director alignment at the pore wall is varied from homeotropic to tangential by pore surface treatment. Several phenomena are uncovered by these studies which probed the weakly first order nematic to isotropic, the continuous smectic-A to nematic and the first order smectic-A to isotropic phase transitions. The specific heat is strongly dependent on the nematic director configuration, and confinement effects are remarkably distinct according to the order of the phase transition. The influence of elastic distortions and surface ordering and disordering effects are evident. Despite considerable departures from bulk behavior with regards to specific heat peaks size, rounding and width, and transition temperature shifts, a bulk-like critical behavior appears to be retained. The formation of smectic translational order within the pores is hindered for those liquid crystals that also possess a nematic phase. The average scalar order parameter temperature dependence is extracted from the specific heat results using a simplified Landau-de Gennes type of model, and is shown to be consistent with nuclear magnetic resonance results.

Orientational effects on confined 5CB

Abstract We report measurements of the heat capacity at the N-I transition for 5CB confined in submicron size cavities. A radically different behavior is observed depending on the orientation of the liquid crystal molecules within the cavities. A comparison with bulk measurements is also made.

Critical behavior at the smectic-A to nematic transition of confined liquid crystal mixtures

Abstract High resolution calorimetry studies at the smectic-A to nematic phase transition confined to the random interconnected voids of Millipore filters were performed. Measurements took place with pure 9CB and (9 + 10)CB mixtures, for bulk and in 0.05 μm void size Millipore samples. This work focuses on the competition between the confinement which affects the bulk-like critical behavior by weakening the coupling between the nematic and smectic order parameters, and the increased coupling that results from the addition of a smectic liquid crystal component.

Confinement and orientational study at liquid crystal phase transitions

Abstract We have studied the heat capacity of the thermotropic liquid crystal, octylcyanobiphenyl (8CB), confined to the nearly cylindrical, 0·2 μm diameter pores of Anopore membranes. Orientation of the nematic director within the pores can be controlled with surface treatment. It is known from NMR measurements that the nematic director is aligned parallel to the pore axis in the untreated membrane. A perpendicular alignment is obtained when the pore surface is treated with lecithin. The second order smectic A to nematic (SA–N) and the weakly first order nematic to isotropic (N–I) phase transitions of 8CB were studied in these pores, for both director orientations, using an AC calorimetry technique. Effects on heat capacity amplitudes, transition temperature shifts, rounding and broadening of these phase transitions will be presented and contrasted with bulk measurements.

The Nematic to Isotropic Phase Transition in an Organic Random Network

Abstract The 4′-n-pentyl-4-cyanobiphenyl (5CB) liquid crystal is confined to the random interconnected voids of fibrous Millipore filter paper. The nematic to isotropic (NI) transition is studied using an ac calorimetry technique on Millipore filters of various void sizes. Even though the nematic correlation length is much smaller than all of the Millipore void sizes employed, substantial differences from the bulk behavior occur at the NI transition.

Surface-Induced Order Detected by Deuteron Nuclear Magnetic Resonance

As many as 30 years ago P. G. de Gennes wrote in the first edition of his book The Physics of Liquid Crystals [1]: “The dual aspects of a nematic phase (liquid-like but uniaxial) are exhibited most spectacularly in the NMR spectrum; the uniaxial symmetry causes certain line splittings, which are absent in the conventional isotropic liquid phase. On the other hand, the lines are relatively narrow; this implies rapid molecular motions and is a natural consequence of the fluidity”.

The Effect of Chirality on the Phase Transitions of Chiral/8CB Mixtures

Abstract We report on a study of mixtures of octylcyanobiphenyl (8CB) and equal amounts of two chiral agents (CE2 and CB15) as a function of chiral pitch (concentration). Bulk studies reveal a widening cholesteric temperature range with increasing chirality. The first order cholesteric-isotropic (Ch-I) transition shifts marginally to lower temperature with increasing chirality while exhibiting a pre-transition feature near its Cp maximum. The smectic-A-cholestenc (SmA-Ch) C, anomaly becomes smaller and continuously shifts to lower temperatures with increasing chirality while exhibiting the unique feature of a discontinuous jump on the low temperature side of the Cp peak. The increased chirality suppresses the smectic formation through twist elastic distortion. Upon Anopore confinement, the Cp maximum of the Ch-I transition increases as compared to bulk with the disappearance of the pre-transition feature; the SmA-Ch transition becomes extremely rounded and suppressed. The transitions shift to lower temper...