Abstract
Understanding the origin of structural ordering in supercooled liquid gallium (Ga) has been a great scientific quest in the past decades. Here, reflective polarized optical microscopy on Ga sandwiched between glasses treated with rubbed polymers reveals the onset of an anisotropic reflection at 120°C that increases on cooling and persists down to room temperature or below. The polymer rubbing usually aligns the director of thermotropic liquid crystals (LCs) parallel to the rubbing direction. On the other hand, when Ga is sandwiched between substrates that align conventional LC molecules normal to the surface, the reflection is isotropic, but mechanical shear force induces anisotropic reflection that relaxes in seconds. Such alignment effects and shear-induced realignment are typical to conventional thermotropic LCs and indicate a LC structure of liquid Ga. Specifically, Ga textures obtained by atomic force and scanning electron microscopy reveal the existence of a lamellar structure corresponding to a smectic LC phase, while the nanometer-thin lamellar structure is transparent under transmission polarized optical microscopy. Such spatial molecular arrangements may be attributed to dimer molecular entities in the supercooled liquid Ga. The LC structure observation of electrically conductive liquid Ga can provide new opportunities in materials science and LC applications.
Highlights
Introduction lived covalentGa dimers, a bond length of near 2.44 Å was attributed to the structural shoulder observed from the moleElemental gallium (Ga) has an unusual mixed covalent and cular dynamics simulations.[2]
We demonstrate a possible liquid crystalline mesophase of supercooled liquid Ga sandwiched between various polymer-coated glass substrates that induce homogeneous or homeotropic alignment in typical liquid crystal (LC) materials
The liquid Ga crystallizes to α-Ga at −51 °C because of the supercooling effect of Ga (Table S1, Supporting Information)
Summary
DSC characterization of sandwiched supercooled liquid Ga shows first-order transitions upon heating and cooling. We performed DSC experiments at higher temperatures up to 400 °C, which did not reveal any first-order phase transition peak in either Ga or EGaIn. In a thin (4 to 6 μm) layer of the supercooled liquid Ga prepared on a clean glass substrate at RT with open-air boundary, a grayish texture is observed using a circularly polarized light in reflective polarized optical microscopy (R-POM) (Figure 1b). When liquid Ga is slightly sheared between clean glass plates with a cell gap of 10 μm, large area domains are split into brighter and dark bands following the uncrossing of the polarizers in different directions This is indicative of domains with the opposite optical activities (see Figure S16, Supporting Information). Supporting Information is available from the Wiley Online Library or from the author
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