Ultrahigh Passive Cooling Power in Hydrogel with Rationally Designed Optofluidic Properties.

The cooling power of a radiative cooler is more than halved in the tropics, e.g., Singapore, because of its harsh weather conditions including high humidity (84% on average), strong downward atmospheric radiation (∼40% higher than elsewhere), abundant rainfall, and intense solar radiation (up to 1200 W/m2 with ∼58% higher UV irradiation). So far, there has been no report of daytime radiative cooling that well achieves effective subambient cooling. Herein, through integrated passive cooling strategies in a hydrogel with desirable optofluidic properties, we demonstrate stable subambient (4-8 °C) cooling even under the strongest solar radiation in Singapore. The integrated passive cooler achieves an ultrahigh cooling power of ∼350 W/m2, 6-10 times higher than a radiative cooler in a tropical climate. An in situ study of radiative cooling with various hydration levels and ambient humidity is conducted to understand the interaction between radiation and evaporative cooling. This work provides insights for the design of an integrated cooler for various climates.

Memory Re-Condensation.

This paper reveals a phenomenon of memory in dropwise condensation in open air. After a first condensation process and complete evaporation of the condensed droplets, further condensations proceed with droplets nucleating at the very places where former droplets evaporated. The origin of this phenomenon is due to the incorporation of airborne salts during the first droplet condensation and its further concentration during droplet evaporation. Salts act as preferential nucleation sites and humidity sinks. The potential impact of this phenomenon on controlled breath figure patterns and plant metabolism is discussed.

Nucleation-enhanced condensation and fast shedding on self-lubricated silicone organogels.

A cross-linked silicone elastomer swollen in silicone oil releases at its surface a thin oil layer, whose thickness slowly increases with time as a consequence of surface energy minimization and residual crosslinking reactions within the polymer matrix. Here, we tune and characterize this oil layer thickness (between 0 and 5 µm) in order to show its quantitative influence on the physical mechanisms at play during water condensation: droplet nucleation and growth, coarsening by menisci-mediated coalescences, and droplet shedding by gravity. We show that continuous nucleation is at the origin of enhanced condensation. Spontaneous replenishment of the oil layer occurs thanks to the storage of oil in the bulk of the swollen elastomer, allowing the same sample to be used for repeated long-lasting condensation experiments.

Lubrication dynamics of swollen silicones to limit long term fouling and microbial biofilms.

Bacterial contamination and biofilm formation on medical devices remain a costly and serious healthcare problem. Silicone (polydimethylsiloxane, PDMS) elastomers are common biomaterials but are susceptible to bacterial surface contamination and biofilm growth. 'Self-lubricated' PDMS elastomers (iPDMS) have the potential to greatly reduce rates of cell attachment, biofilm formation and infection. Cross-linked PDMS elastomers immersed in PDMS oil swell to an equilibrium concentration to form a swollen network, and then form a surface liquid layer through syneresis. Herein we have measured the swelling and syneresis kinetics as a function of time, viscosity (1.5 to 10 cSt), and cross-linking density to optimize the surface lubricant layer formation, and resistance to biofouling. The lubricant layer thickness was measured in situ (optical profilometry and AFM) for flat and micro-textured surfaces, as a function of time and swelling ratio, to be in a range from 0.1 to 1 µm, and continuously increases with time. We show this continuous generation is likely due to a gradual, dynamic re-structuring of the elastomer network. Long term antifouling properties of (10 cSt) iPDMS were tested for Pseudomonas aeruginosa growth in a flow culture bioreactor, and after 30 d showed a 103 to 104 reduction of bacterial cell density for iPDMS compared to conventional PDMS elastomers. This long term performance and non-specific activity makes them highly suitable for biomedical devices, such as urinary catheters.

Mechanical deformation of elastomer medical devices can enable microbial surface colonization.

Elastomers such as silicone are common in medical devices (catheters, prosthetic implants, endoscopes), but they remain prone to microbial colonization and biofilm infections. For the first time, our work shows that rates of microbial surface attachment to polydimethylsiloxane (PDMS) silicone can be significantly affected by mechanical deformation. For a section of bent commercial catheter tubing, bacteria (P. aeruginosa) show a strong preference for the 'convex' side compared to the 'concave' side, by a factor of 4.2. Further testing of cast PDMS materials in bending only showed a significant difference for samples that were manually wiped (damaged) beforehand (1.75 × 104 and 6.02 × 103 cells/mm2 on the convex and concave sides, respectively). We demonstrate that surface microcracks in elastomers are opened under tensile stress (convex bending) to become 'activated' as sites for microbial colonization. This work demonstrates that the high elastic limit of elastomers enables these microcracks to reversibly open and close, as 'dynamic defects'. Commercial catheters have relatively high surface roughness inherent to manufacturing, but we show that even manual wiping of newly-cast PDMS is sufficient to generate surface microcracks. We consider the implication for medical devices that feature sustained, surgical, or cyclic deformation, in which localized tensile conditions may expose these surface defects to opportunistic microbes. As a result, our work showcases serious potential problems in the widespread usage and development of elastomers in medical devices.

Simultaneous electrospinning and electrospraying: a straightforward approach for fabricating hierarchically structured composite membranes.

We present here for the first time a simple method for micropatterning nonwoven composite membranes. The approach is based on the simultaneous electrospraying of microparticles and electrospinning of nanofibers from different polymer solution feeds (polyethylene glycol and poly(D,L-lactide)) on a common support. The mechanism of self-organization between fibers and particles into hierarchical honeycomb-like structures, as well as the evolution of the later as a function of the thickness of the composite, is investigated. We demonstrate that aggregates of particles, leading to a nonuniform distribution of the electrostatic field near the collector, are necessary to form the self-organized composite. Furthermore, it is shown that the specific dimensions of the generated patterns can be controlled by tuning the flow rate of electrospraying. The obtained composite mat exhibits a multilevel porous structure, with pore sizes ranging from few up to several hundreds of micrometers. Finally, it is shown that the microparticles can be selectively leached, allowing the production of a monocomponent membrane and retaining the hierarchical organization of the nanofibers suitable for biomedical and filtration applications.