Texture Changes inside Smectic-C Droplets in Azobenzene Langmuir Monolayers

Abstract

Preparation of Langmuir monolayers of a mixture of trans- and cis-isomers of an azobenzene derivative, 4-[4-[(4-octylphenyl)azo]phenoxy]butanoic acid, results in the segregation of birefringent trans-isomer domains embedded in an isotropic medium of cis-isomers. Brewster angle microscopy observations allow us to identify different textures inside the domains depending on surface pressure, temperature, and domain size. The evolution of the monolayer in the dark, from initial droplets formed after spreading to a stable stripe texture, is described. The dynamics of domain coalescence and some morphological transitions induced by temperature and surface pressure changes are also discussed. A simple theoretical model is included to supplement some of these experimental observations.

Stirring competes with chemical induction in chiral selection of soft matter aggregates

“…Using polarizing reflection microscopy at the Brewster angle (BAM), we are able to detect that, inside these discs, surfactant molecules in the trans configuration organize with a uniform tilt with respect to the surface normal, and a spatially varying in-plane projection, which can be characterized by a two-dimensional vector field, c(x, y). Reflectivity patterns inside the discs unambiguously reveal that, on average, molecules are locally tilted along a direction parallel to the domain boundary (bend-like ordering) 37 , either clockwise (CW) or counter-clockwise (CCW), that is, c ¼ ±r  s, where r is the radial coordinate with respect to the domain centre, s is the unit vector normal to the air/water interface and '  ' denotes the vector product. This defines the two enantiomorphic configurations of the orientational order that are equally represented in the absence of any chiral influence (Fig.…”

“…The emergence of chiral structures is documented in insoluble monolayers at the air/water interface (Langmuir monolayer), both involving chiral 35 and non-chiral components 36,37 . Here we work with an azobenzene-based photoresponsive achiral amphiphile, Azo1 (see Methods), known to feature spontaneous chiral resolution.…”

Stirring competes with chemical induction in chiral selection of Langmuir monolayer domains

“…After the elapsed stirring time of 35 minutes, roughly half the surface is covered by the transrich domains. BAM reflectivity patterns inside the disks unambiguously reveal that, on average, molecules are locally tilted by the same angle along a direction parallel to the domain boundary (bend-like ordering) [7], either clockwise (CW) or counter-clockwise (CCW). This defines the two enantiomorphic configurations of the orientational order that are equally represented in absence of any chiral influence (Figure 1).…”

“…In Langmuir monolayers, it is well-known that the use of enantiomeric chiral surfactants typically leads to the formation of ordered domains with enantiomorphic (mirror-symmetric) chiral shapes [5]. It has also been observed that achiral surfactants can lead to the formation of ordered domains of achiral shapes but with chiral inner textures of molecular order [6,7]. In these cases, symmetry conservation demands that domains with enantiomorphic textures be equally probable.…”

Probing Elastic Anisotropy from Defect Dynamics in Langmuir Monolayers

“…The monolayer is a cis and trans mixture of the photosensitive azobenzene amphiphile 8Az3COOH [9]. Monolayers feature isolated birefringent trans-rich domains (henceforth droplets) surrounded by an isotropic cis-rich medium [10]. Mesoscale organization inside the droplets is probed with a custom-built Brewster angle microscope (BAM) [11].…”