Printed electronics have led to new possibilities in the detection and quantification of a wide range of molecules important for medical, biotechnological, and environmental fields. The integration with microfluidics is often adopted to avoid hand-deposition of little volumes of reagents and samples on miniaturized electrodes that strongly depend on operator’s skills. Here we report design, fabrication and test of an easy-to-use electrochemical sensor platform with microfluidics entirely realized with Aerosol Jet Printing (AJP). We printed a six-electrochemical-sensors platform with AJP and we explored the possibility to aerosol jet print directly on it a microfluidic structure without any support material. Thus, the sacrificial material removal and/or the assembly with sensors steps are avoided. The repeatability observed when printing both conductive and ultraviolet (UV)-curable polymer inks can be supported from the values of relative standard deviation of maximum 5% for thickness and 9% for line width. We designed the whole microfluidic platform to make the sample deposition (20 μL) independent from the operator. To validate the platform, we quantified glucose at different concentrations using a standard enzyme-mediated procedure. Both mediator and enzyme were directly aerosol jet printed on working electrodes (WEs), thus the proposed platform is entirely fabricated by AJP and ready to use. The chronoamperometric tests show limit of detection (LOD) = 2.4 mM and sensitivity = 2.2 ± 0.08 µA/mM confirming the effectiveness of mediator and enzyme directly aerosol jet printed to provide sensing in a clinically relevant range (3–10 mM). The average relative standard inter-platform deviation is about 8%. AJP technique can be used for fabricating a ready-to-use microfluidic device that does not need further processing after fabrication, but is promptly available for electrochemical sample analysis.

The modeling of the viability decay of viruses in sessile droplets is addressed considering a droplet sitting on a smooth surface characterized by a specific contact angle. To investigate, at prescribed temperature, how surface energy of the material and ambient humidity cooperate to determine the virus viability, we propose a model which involves the minimum number of thermodynamically relevant parameters. In particular, by considering a saline water droplet (one salt) as the simplest approximation of real solutions (medium and natural/artificial saliva), the evaporation is described by a first-order time-dependent nonlinear differential equation properly rearranged to obtain the contact angle evolution as the sole unknown function. The analyses were performed for several contact angles and two typical droplet sizes of interest in real situations by assuming constant ambient temperature and relative humidity in the range 0–100%. The results of the simulations, given in terms of time evolution of salt concentration, vapor pressure, and droplet volume, elucidate some previously not yet well-understood dynamics, demonstrating how three main regimes—directly implicated in nontrivial trends of virus viability and to date only highlighted experimentally—can be recognized as the function of relative humidity. By recalling the concept of cumulative dose of salts (CD), to account for the effect of the exposition of viruses to salt concentration on virus viability, we show how the proposed approach could suggest a chart of a virus fate by predicting its survival time at a given temperature as a function of the relative humidity and contact angle. We found a good agreement with experimental data for various enveloped viruses and predicted in particular for the Phi6 virus, a surrogate of coronavirus, the characteristic U-shaped dependence of viability on relative humidity. Given the generality of the model and once experimental data are available that link the vulnerability of a certain virus, such as SARS-CoV-2, to the concentrations of salts or other substances in terms of CD, it is felt that this approach could be employed for antivirus strategies and protocols for the prediction/reduction of human health risks associated with SARS-CoV-2 and other viruses.

Cactaceae have adapted to harsh environments by resisting intense solar radiation, retaining and collecting water. Some cacti species have hairs on them along with distinctive spines to serve different functions. In this study, we characterized the Old Man of Andes cactus (Oreocereus trolli). We examined the surface morphology and estimated roughness and adhesion force of both spines and hairs. They both consist of longitudinal microgrooves. The condensation experiments carried out on spines showed that there is a particular phenomenon of distant coalescence (DC): smaller droplets flow totally or partially into larger ones through the microgrooves with consequent accumulation of water in a few large drops. An earlier study (Bintein et al., 2019) has shown that artificial micro-grooved surfaces that exhibit DC are more efficient than flat ones at collecting and sliding dew, and thus cactus spines could act as soil dew conveyors. The agreement between our analytical model and experimental data verifies that the flow is driven by the Laplace pressure difference between the drops. This allowed us to obtain a general criterion for predicting the total or partial emptying of the smaller drops as a function of the dynamic contact angles of a surface. With the Wilhelmy method, we determined that hairs are less hydrophilic than spines. We also estimated the mechanical properties of both the spines and hairs to evaluate their possible role in physical defense. This study aids in better understanding the physical attributes and the condensation interaction with microstructures, and suggests some functional roles.

Fascinated by the purple color, water-repellent, and self-cleaning properties of Cotinus coggygria Scop. leaves, we studied their morphology, wetting, and condensation frosting. Wax nanotubules confer high contact angles, enabling coalescence-induced condensation droplet (out-of-plane) jumping, which, as known, contributes to slowing down frost. Another type of movement—this time in-plane—becomes predominant in reducing the frosting velocity ($v_{frost}$) within a sub-cooling temperature range. Specifically, supercooled droplets slide toward the frost bridges upon contact, moving in the opposite direction to frost propagation. Between −11 and −2°C, Sliding on Frost (SoF) shifts from being rare to very frequent, reducing $v_{frost}/v_{bridge}$ from approximately 4 to 1, respectively. Using high-speed microscopy, we observed that the advancing contact angle of supercooled water on ice decreases with temperature. We describe the primary role of this behavior in SoF with a model that accounts for the forces involved and explains the observed transition.

Surfaces capable of delaying the frosting passively and facilitating its removal are highly desirable in fields where ice introduces inefficiencies and risks. Coalescence‐induced condensation droplets jumping (CICDJ), enabled on highly hydrophobic surfaces, is already exploited to slow down the frosting but it is insufficient to completely eliminate the propagation by ice‐bridging. The study shows here how the self‐ejection of single condensation droplets can fully frustrate all the ice bridges, resulting in a frost velocity lower than 0.5 µm s⁻¹ and thus falling below the current limits of passive surfaces. Arrays of truncated microcones, covered by uniformly hydrophobic nanostructures, enable individual condensation droplets to grow and self‐propel toward the top of the microstructures and then to self‐eject once a precise volume is reached. The independency of self‐ejection on the neighbor droplets allows a fine control of the droplets size and distance distributions and thus the ice‐bridging frustration. The truncated microcones with the smallest heads area fraction maximize the percentage of self‐ejecting droplets and minimize the frost velocity. The ice bridges frustration also implies a small frost area coverage, highly desirable in aeronautics and thermal machines.