De Novo Determination of Molecular Orientation at Interfaces through Multimode Polarization-Dependent Sum-Frequency Generation Spectroscopy

Many essential processes occur at soft interfaces, from chemical reactions on aqueous aerosols in the atmosphere to biochemical recognition and binding at the surface of cell membranes. The spatial arrangement of molecules specifically at these interfaces is crucial for many of such processes. The accurate determination of the interfacial molecular orientation has been challenging due to the low number of molecules at interfaces and the ambiguity of their orientational distribution. Here, we combine phase- and polarization-resolved sum-frequency generation spectroscopy to obtain the molecular orientation at the interface. We extend an exponentially decaying orientational distribution to multiple dimensions, which, in conjunction with multiple SFG data sets obtained from the different vibrational modes, allows us to determine molecular orientation. We apply this new approach to formic acid molecules at the air-water interface. The inferred orientation of formic acid agrees very well with ab initio molecular dynamics data. The phase-resolved SFG multimode analysis scheme using the multi-dimensional orientational distribution thus provides a universal approach for obtaining the interfacial molecular orientation.

Small-angle X-ray scattering from GaN nanowires on Si(111): facet truncation rods, facet roughness and Porod's law

Small-angle X-ray scattering from GaN nanowires grown on Si(111) is measured in the grazing-incidence geometry and modelled by means of a Monte Carlo simulation that takes into account the orientational distribution of the faceted nanowires and the roughness of their side facets. It is found that the scattering intensity at large wavevectors does not follow Porod's law I(q) ∝ q −4. The intensity depends on the orientation of the side facets with respect to the incident X-ray beam. It is maximum when the scattering vector is directed along a facet normal, reminiscent of surface truncation rod scattering. At large wavevectors q, the scattering intensity is reduced by surface roughness. A root-mean-square roughness of 0.9 nm, which is the height of just 3–4 atomic steps per micrometre-long facet, already gives rise to a strong intensity reduction.

Orientational Order in the Splay Nematic Ground State

<p>Modulated nematic liquid crystal phases, which lack positional order but have some periodic variation in the direction of average orientation present in a classical nematic, have attracted significant interest. In the recently discovered splay nematic (N_S) phase the average orientational order is augmented with a periodic splay deformation of orientation perpendicular to the director. We use X-ray scattering experiments to measure the orientational order parameters in the nematic (N) and splay nematic (N_S) phases of the liquid crystalline material RM734. The degree of orientational order is somewhat larger in the N_S phase than in the preceding nematic and temperature dependent. We reconstruct the orientational distribution function and find it to be nematic-like in the N_S phase, indicating the change in orientation between neighbouring molecules due to the splay modulation is very small. A small splay angle implies that the splay modulation period is larger than the few tens of nanometers originally envisaged. The method described herein can be used to assist in unambiguous identification of the splay-nematic phase.</p>

Orientational Order in the Splay Nematic Ground State

<p>Modulated nematic liquid crystal phases, which lack positional order but have some periodic variation in the direction of average orientation present in a classical nematic, have attracted significant interest. In the recently discovered splay nematic (N_S) phase the average orientational order is augmented with a periodic splay deformation of orientation perpendicular to the director. We use X-ray scattering experiments to measure the orientational order parameters in the nematic (N) and splay nematic (N_S) phases of the liquid crystalline material RM734. The degree of orientational order is somewhat larger in the N_S phase than in the preceding nematic and temperature dependent. We reconstruct the orientational distribution function and find it to be nematic-like in the N_S phase, indicating the change in orientation between neighbouring molecules due to the splay modulation is very small. A small splay angle implies that the splay modulation period is larger than the few tens of nanometers originally envisaged. The method described herein can be used to assist in unambiguous identification of the splay-nematic phase.</p>

Orientation Distributions of Cellulose Nanofibrils and Nanocrystals in Confined Flow

The pursuit of sustainable and environmentally friendly materials has been driving a tremendous interest in biobased alternatives in the last decade. Nanocellulose has been widely seen as a prime contender due to its impressive properties as well as being abundant and biodegradable. Recently, it has been demonstrated how nanocellulosic materials can be hydrodynamically aligned in flows and assembled continuously into materials with tunable macroscopic properties. However, the aligning mechanisms of the highly entangled system of elongated nanoparticles in different flow situations still remain largely unknown. Here, we investigate the orientation distributions of cellulose nanofibrils and nanocrystals (CNF and CNC) in a straight quadratic channel at various flow rates using small-angle X-ray scattering (SAXS), where CNF and CNC are aligned by strong shear flow close to the walls. In dilute systems, CNC behave as Brownian ellipsoids, while at semi-dilute concentrations there seems to be a limit to how high alignment of CNF and CNC can be achieved in a shear dominated flow even though particle interactions clearly aid in aligning the system at low flow rates. Furthermore, we show how some essential parameters in the orientational distribution can be obtained with polarized optical microscopy.

Orientation Distributions of Cellulose Nanofibrils and Nanocrystals in Confined Flow

The pursuit of sustainable and environmentally friendly materials has been driving a tremendous interest in biobased alternatives in the last decade. Nanocellulose has been widely seen as a prime contender due to its impressive properties as well as being abundant and biodegradable. Recently, it has been demonstrated how nanocellulosic materials can be hydrodynamically aligned in flows and assembled continuously into materials with tunable macroscopic properties. However, the aligning mechanisms of the highly entangled system of elongated nanoparticles in different flow situations still remain largely unknown. Here, we investigate the orientation distributions of cellulose nanofibrils and nanocrystals (CNF and CNC) in a straight quadratic channel at various flow rates using small-angle X-ray scattering (SAXS), where CNF and CNC are aligned by strong shear flow close to the walls. In dilute systems, CNC behave as Brownian ellipsoids, while at semi-dilute concentrations there seems to be a limit to how high alignment of CNF and CNC can be achieved in a shear dominated flow even though particle interactions clearly aid in aligning the system at low flow rates. Furthermore, we show how some essential parameters in the orientational distribution can be obtained with polarized optical microscopy.

Impact of Weak Nanoparticle Induced Disorder on Nematic Ordering

Dilute mixtures of nanoparticles (NPs) and nematic liquid crystals (LCs) are considered. We focus on cases where NPs enforce a relatively weak disorder to the LC host. We use a Lebwohl-Lasher semi-microscopic-type modeling where we assume that NPs effectively act as a spatially-dependent external field on nematic spins. The orientational distribution of locally favoured “easy” orientations is described by a probabilistic distribution function P. By means of a mean field-type approach, we derive a self-consistent equation for the average degree of nematic uniaxial order parameter S as a function of the concentration p of NPs, NP-LC coupling strength and P. Using a simple step-like probability distribution shape, we obtain the S(p) dependence displaying a crossover behaviour between two different regimes which is in line with recent experimental observations. We also discuss a possible origin of commonly observed non-monotonous variations of the nematic-isotropic phase temperature coexistence width on varying p.