OB1 MK4 - Microfluidic Flow Controller

The optical properties of liquid crystals serve as the basis for display, diagnostic, and sensing technologies. Such properties are generally controlled by relying on electric fields. In this work, we investigate the effects of microfluidic flows and acoustic fields on the molecular orientation and the corresponding optical response of nematic liquid crystals. Several previously unknown structures are identified, which are rationalized in terms of a state diagram as a function of the strengths of the flow and the acoustic field. The new structures are interpreted by relying on calculations with a free energy functional expressed in terms of the tensorial order parameter, using continuum theory simulations in the Landau-de Gennes framework. Taken together, the findings presented here offer promise for the development of new systems based on combinations of sound, flow, and confinement.

In the age of connected mobility, monitoring volatile organic compounds (VOCs) inside the vehicle plays a pivotal role in effectively controlling in-cabin air quality conditions. In this context, it is imperative to develop new monitoring devices that not only detect VOCs at low concentrations, but also can discern those compounds that pose a major risk to the health and comfort of occupants. Microfluidic gas detectors have recently emerged as a promising solution for the cost-effective discretization of VOCs in interior environments. Although microfluidic gas detectors may not yet reach the accuracy of other analytical methods, these devices offer a simple and compact design, can work at room-temperature, and do not require a carrier gas to operate; which represent a clear competitive advantage for their implementation in the vehicle. This work intends to study the feasibility of a 3D-printed microfluidic gas detector for the semi-selective detection of VOCs inside the vehicle cabin. In particular, this research aims to analyze whether the selectivity of these devices can be improved by (i) optimizing the thickness of the polymer layer on the walls of the microchannel, and (ii) understanding the influence of the chemical affinity between VOCs and the coating polymer. Empirical results reveal that thicker polymer films enable to enhance and expand the retention capabilities of the microchannel. Additionally, the Hansen solubility parameters are presented as a suitable tool to analyze and determine how polymer-analyte affinity, in combination with other intrinsic properties of analytes, influences the elution of contaminants from the microfluidic channel as well.

Confinement in microenvironments occurs in many natural systems and technological applications. However, little is known about the behaviour of the immersed nanoparticles. In this work we show that their diffusion, electro-orientation and electric field induced polarization can be determined through electric birefringence experiments. We analyze aqueous dispersions of silver nanowires and clay particles confined inside microdroplets. We have observed that confinement reduces the amount of particles that can be oriented by the external electric field. However, the polarizability of the oriented particles is not affected by the presence of the oil/water boundary, and it is the same as in unbounded media, which agrees with the fact that the electric polarization and related phenomena are short-ranged.

Wetting and water absorption of hydrogels is often encountered in many applications. We seek to understand how wetting behavior can be affected by the time-dependent swelling of hydrogels. We measured the advancing contact angles of water droplets on hydrogels of varying thicknesses where thicker gels absorbed water more slowly. We also observed that, above a threshold advancing speed, water droplets would collapse into a lower contact angle state on the surface. We hypothesized that this collapse threshold speed is a result of competition between the poroelastic diffusion of water into the gel and the advance of the spreading droplet, the thickness of the surface, and the diffusion of water into the gel. Taking the ratio of the diffusion and advancing timescales results in a Peclet number with gel thickness as a characteristic length scale. Our results show that above a Peclet number of around 40, droplets will collapse on the surface across all gel thicknesses, confirming our hypothesis. This work provides simple insight to understand a complex time-dependent wetting phenomenon for a widely used hydrogel.