Super-enhanced evaporation of droplets from porous coatings

HYPOTHESIS

The addition of a thin, hydrophilic, porous, coating to an impermeable solid will lead to more rapid evaporation of liquid droplets that impinge on the solid. The droplet will imbibe quickly, but the progress normal to the interface will be limited to the thickness of the coating, and therefore the liquid will spread laterally into a broad disk to expose a large liquid-vapor interface for evaporation.

EXPERIMENTS

Liquid droplets of volume 2.5-25 µL were placed on solids and then both the mass and area of each droplet were monitored over time. We compared data for smooth, impermeable hydrophilic glass to the same glass that was coated in thin (35-109 µm) porous, hydrophilic-glass layer fabricated from glass beads.

FINDINGS

The droplet was imbibed (wicked) into the coating within seconds, and the liquid spread laterally to form a thin, broad, disk. Critically, evaporation of a droplet was enhanced by a factor of 7-8 on the thin coating. The evaporation rate was not proportional to the reciprocal thickness of the coating. The ability to enhance evaporation of small droplets on a solid may have practical applications, for example, in speeding the death of microbes.

Droplet evaporation on porous fabric materials

Droplet evaporation on porous materials is a complex dynamic that occurs with spontaneous liquid imbibition through pores by capillary action. Here, we explore water dynamics on a porous fabric substrate with in-situ observations of X-ray and optical imaging techniques. We show how spreading and wicking lead to water imbibition through a porous substrate, enhancing the wetted surface area and consequently promoting evaporation. These sequential dynamics offer a framework to understand the alterations in the evaporation due to porosity for the particular case of fabric materials and a clue of how face masks interact with respiratory droplets.

Measurement and Modeling of Spontaneous Capillary Imbibition in Coal

Coal is a typical dual-porosity medium. The implementation process of water invasion technology in coal is actually a process of spontaneous imbibition of external water. To obtain a model of spontaneous capillary imbibition in coal, the spontaneous imbibition of water in coal samples with different production loads is conducted experimentally. Due to the coal particle deformation and the cohesive forces, the porosity and maximum diameter decrease gradually with increasing pressing loads. Due to the filling effects and occupying effects, the proper particle grading can reduce the porosity and tortuosity. The Comiti model can be used to describe the tortuosity. The tortuosity increases with decreasing porosity. The smaller the porosity, the smoother the surface of the coal sample. The contact angle is negatively correlated with the surface roughness. The fractal dimension decreases with increasing pressing load. The difference in the pore characteristics between particles is the main reason for the difference in the fractal dimension. The proposed model of spontaneous capillary imbibition in coal is consistent with the experimental data. The implications of this study are important for understanding the law of spontaneous imbibition in coal and the displacement of gas by spontaneous capillary imbibition in coal, which is important for optimizing the parameters of coal seam water injection.

Imbibition of Femtoliter-Scale DNA-Rich Aqueous Droplets into Porous Nylon Substrates by Molecular Printing

This work presents the first reported imbibition mechanism of femtoliter (fL)-scale droplets produced by microchannel cantilever spotting (μCS) of DNA molecular inks into porous substrates (hydrophilic nylon). Differently from macroscopic or picoliter droplets, the downscaling to the fL-size leads to an imbibition process controlled by the subtle interplay of evaporation, spreading, viscosity, and capillarity, with gravitational forces being quasi-negligible. In particular, the minimization of droplet evaporation, surface tension, and viscosity allows for a reproducible droplet imbibition process. The dwell time on the nylon surface permits further tuning of the droplet lateral size, in accord with liquid ink diffusion mechanisms. The functionality of the printed DNA molecules is demonstrated at different imbibed oligonucleotide concentrations by hybridization with a fluorolabeled complementary sequence, resulting in a homogeneous coverage of DNA within the imbibed droplet. This study represents a first step toward the μCS-enabled fabrication of DNA-based biosensors and microarrays into porous substrates.

Wetting of Inkjet Polymer Droplets on Porous Alumina Substrates

The resolution of inkjet printing technology is determined by wetting and evaporation processes after the jet drop contacts the substrate. Here, the wetting of different picoliter solubilized polymer droplets jetting onto one-end-closed porous alumina was investigated. The selected polymers are commonly used in inkjet ink. The synergistic effects of the hierarchical structure and substrate surface modification were used to control the behavior of polymer-based ink drops. A model that invokes the effect of surface tension was applied to calculate the amount of polymer solution penetrating into the pores. The calculation corroborates experimental observations and shows that the volume of polymer solution in the pores increases with an increase in pore radius and depth, resulting in less solution remaining on the substrate surface. The structure of the porous substrate coupled with intrinsic polymer properties and surface modifications all contribute to the resolution that can be achieved via inkjet printing.