(73e) Liquid Crystal-Infused Porous Surfaces with Molecular Order-Dependent Slipperiness and Cargo Release

The design of open surface microfluidics that enable orthogonal control of liquid mobility and chemical compositions is critical for devising the next generation of microfluidic platforms that will find use in applications across chemical, environmental, and biomedical fields. To achieve these desirable functionalities, extensive studies have demonstrated stimuli-responsive liquid mobility on open fluidic platforms based on either micro/nanoscale topographical surfaces or water-immiscible liquid-coated surfaces. However, methods of manipulating droplets’ chemical compositions tend to rely upon chemical adsorption directly from the underlying surface, which has been shown to subsequently pin droplets to the surface and render them immobile, as illustrated in Scheme 1A. This intrinsic coupling of droplets’ chemical composition and mobility greatly weaken the robustness of conventional open surface microfluidic systems and hinders their use in real-world applications, presenting a substantial challenge that must be overcome to realize the full potential of next-generation open microfluidics.

Thermotropic liquid crystals (LCs) are a particularly promising class of anisotropic fluids that produce a remarkable diversity of colloidal and interfacial phenomena with unprecedented complexities and functionalities. Due to their intrinsic properties, including positional order and orientational order in various mesophases, immobilized LCs have been exploited for a variety of applications, including sensing chemicals and activating the release of cargo. We herein propose that these intrinsic properties may enable the design of LC-based open surface microfluidics that can independently manipulate both the mobility and chemical compositions of resting droplets, as illustrated in Scheme 1B. Though promising, the design of LC-based open surface microfluidics has not yet been achieved due to water droplet-induced dewetting of LC films coated on conventional hydrophobically modified substrates.

In this presentation, we report the design of an LC-based open surface microfluidic platform that enables the independent manipulation of the mobility and chemical compositions of droplets. Specifically, we use porous LC polymeric networks to stabilize thermotropic LC mesogens to overcome the aforementioned issue of water-induced LC dewetting. We find that the mobility of water droplets on LC-based surfaces depends only on the positional order of the LC: water droplets become highly pinned at LC surfaces in the smectic A phase, whereas droplets can freely slide without pinning at LC surfaces in both the nematic and isotropic phases. Moreover, we experimentally and theoretically demonstrate that the mesogenic orientational order of the LC surface plays a pivotal role in the release of chemicals from the LC surface to droplets. Finally, we demonstrate that as a consequence of the inherent decoupling between a droplet’s mobility and the release of cargo from the LC, LC-based open surface microfluidic platforms can capture and precipitate heavy metal ions in droplets of water at LC surfaces without hindering droplets’ mobilities. Our work provides novel design principles for fabricating anisotropic liquid-based open surface microfluidics that enable promising applications including liquid droplet-based chemical synthesis and medical diagnostics.

Scheme 1. (A) Chemicals released from conventional functional surfaces, causing water droplets to become pinned. (B) Independent manipulation of liquid mobility and chemical composition at LC surfaces.