Monday 30 May 2022

Emmanouil Anyfantakis
Experimental Soft Matter Physics Group, Department of Physics & Materials Science, Université du Luxembourg, Luxembourg
Plasmonic liquid mirrors and photonic liquid marbles: how to exploit colloid particles at interfaces for making self-organized functional materials
Room A2 (A115-A117) of CSD Building (ground floor, north wing).

The adsorption and organization of colloid particles at fluid interfaces is a thoroughly investigated research field that continues to attract the vivid interest of scientists and engineers. On the one hand, particles at interfaces constitute an excellent platform for fundamental investigations of physical phenomena occurring in two dimensions. On the other hand, such systems offer a plethora of application opportunities for preparing soft, functional, and often reconfigurable materials. In this talk, we discuss two examples where particles decorating an air-water interface lead, directly or indirectly, to macroscopic materials with tailored optical functionalities that respond to various external stimuli.

In the first example, we discuss plasmonic liquid mirrors (PLMs) made from noble metal nanoparticles (NPs) that self-assemble at a fluid interface. Due to surface plasmon resonance, close-packed NP arrays show metallic properties like high reflectance, while maintaining fluid-like rheological properties. Although PLMs could be useful for reconfigurable optical elements, they have been formed only at water- organic solvent interfaces. Here, we follow a strategy we recently developed to induce both the adsorption of Au NPs from the bulk of an aqueous suspension to its interface with air, and the interfacial self-assembly of the NPs, without any additional solvent involved. We achieve this by utilizing a cationic surfactant, at a concentration range where minimal surfactant adsorption onto the anionic NPs leaves their surface charge unaltered. Surfactants instead adsorb mostly at the air-water interface and by neutralizing it, allow suspended particles to overcome the electrostatic adsorption barrier and attach onto the interface. The interaction potential of the interfacial particles is dictated by double-layer electrostatic repulsion and gravity, which concentrates particles at the center of the concave air-water interface. By varying the surfactant concentration, we modulate the electrostatic repulsion and in turn tune particle-particle interactions. This results into Au NP arrays ranging from close-packed 2D crystals, to 2D and 3D gel-like structures. The structure of the NP arrays defines the optical properties of the NP-laden interface; dense arrays of Au NPs give rise to highly reflective interfaces, which act as good PLMs.

In the second example, we discuss liquid marbles (LMs), sessile drops of liquid coated by lyophobic nanoparticles. These soft objects have been mainly exploited for transporting the encapsulated liquid across various substrates with no leakage. The potential of LMs to serve as mini-reactors for materials synthesis, although realized soon after their discovery, it has been exploited very little and mostly for “conventional” chemical reactions. Here, we present a new concept, where a LM acts as a miniature platform for inducing, monitoring, and controlling the self-organization of a bioderived polymer into a cholesteric liquid crystal with tailored optical properties. We use aqueous solutions of hydroxypropyl cellulose, which form short-pitch cholesteric liquid crystalline phases above a critical concentration. At higher concentrations, the cholesteric pitch reduces to sub-micrometer values and the material displays structural coloration due to Bragg reflection. Considering this, we prepare LMs in the biphasic regime where an isotropic and a cholesteric phase coexist, and by immersing the LMs in an organic solvent with poor water miscibility, we slowly extract the desired amount of water. This allows the polymer chains to self-assemble into a fully cholesteric structure, the pitch of which can be programmed by tuning the final polymer concentration in the LM. This results in LMs with selective Bragg reflection that can be tailored to be anywhere in the visible. Interestingly, the optical response of the cholesteric LMs is very sensitive to various external stimuli. For instance, LMs respond with color changes detectable by the unaided eye, to changes in temperature, presence of toxic chemicals in their environment, and mechanical compression.

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