Droplet evaporation on porous fabric materials

Microporous electrospun nonwovens combined with green solvents for the selective peel-off of thin coatings from painting surfaces

Over the last few decades, significant research efforts have been devoted to developing new cleaning systems aimed at preserving cultural heritage. One of the main objectives is to selectively remove aged or undesirable coatings from painted surfaces while preventing the cleaning solvent from permeating and engaging with the pictorial layers. In this work, we propose the use of electrospun polyamide 6,6 nonwovens in conjunction with a green solvent (dimethyl carbonate). By adjusting the electrospinning parameters, we produced three distinct nonwovens with varying average fiber diameters, ranging from 0.4 μm to 2 μm. These samples were characterized and tested for their efficacy in removing dammar varnish from painted surfaces. In particular, the cleaning process was monitored using macroscale PL (photoluminescence) imaging in real-time, while post-application examination of the mats was performed through scanning electron microscopy. The solvent evaporation rate from the different nonwovens was evaluated using gravimetric analysis and Proton Transfer Reaction- Time-of-Flight. It was observed that the application of the nonwovens with small or intermediate pore sizes for the removal of the terpenic varnish resulted in the swollen resin being absorbed into the mats, showcasing a peel-off effect. Thus, this protocol eliminates the need for further potentially detrimental removal procedures involving cotton swabs. The experimental data suggests that the peel-off effect relates to the microporosity of the mats, which enhances the capillary rise of the swollen varnish. Furthermore, the application of these systems to historical paintings underwent preliminary validation using a real painting from the 20th century.

Heat Transfer by Sweat Droplet Evaporation

Sweating regulates the body temperature in extreme environments or during exercise. Here, we investigate the evaporative heat transfer of a sweat droplet at the microscale to unveil how the evaporation complexity of a sweat droplet would affect the body's ability to cool under specific environmental conditions. Our findings reveal that, depending on the relative humidity and temperature levels, sweat droplets experience imperfect evaporation dynamics, whereas water droplets evaporate perfectly at equivalent ambient conditions. At low humidity, the sweat droplet fully evaporates and leaves a solid deposit, while at high humidity, the droplet never reaches a solid deposit and maintains a liquid phase residue for both low and high temperatures. This unprecedented evaporation mechanism of a sweat droplet is attributed to the intricate physicochemical properties of sweat as a biofluid. We suppose that the sweat residue deposited on the surface by evaporation is continuously absorbing the surrounding moisture. This route leads to reduced evaporative heat transfer, increased heat index, and potential impairment of the body's thermoregulation capacity. The insights into the evaporative heat transfer dynamics at the microscale would help us to improve the knowledge of the body's natural cooling mechanism with practical applications in healthcare, materials science, and sports science.

Two-phase Porous Media Flow Model Based on the Incompressible Navier-Stokes Equation

Two-phase porous media flow is important in many applications from drug delivery to groundwater diffusion and oil recovery and is of particular interest to biomedical diagnostic test developers using cellulose and nitrocellulose membranes with limited fluid sample volumes. This work presents a new two-phase porous media flow model based on the incompressible Navier-Stokes equation. The model aims to address the limitations of existing methods by incorporating a partial saturation distribution in porous media to account for limited fluid volumes. The basic parameters of the model are the pore size distribution and the contact angle. To validate the model, we solved five analytical solutions and compared them to corresponding experimental data. The experimentally measured penetration length data agreed with the model predictions, demonstrating model accuracy. Our findings suggest that this new two-phase porous media flow model can provide a valuable tool for researchers developing fluidic assays in paper and other porous media.

Evaluating Droplet Survivability on Face Masks with X-ray Microtomography

When a person talks, coughs, or sneezes, respiratory droplets are expelled and inevitably land on several surfaces, representing a route for respiratory disease transmission. Here, face masks act as a barrier by obstructing the passage of droplets during exhalation and inhalation. Being constantly exposed to respiratory events and carrying droplet residue, understanding the evaporation and absorption dynamics for tiny droplets on face masks and the fate of viral particle deposition is necessary to analyze the contamination risk. We explore the ideal design for masks from the interaction of mask surfaces with surrogate respiratory droplets by X-ray microscopy and microtomography. We show that the respiratory droplet survivability is significantly reduced in masks with a hydrophilic surface where absorption takes place, leading to a reduction of the postevaporation droplet residue at the mask surface compared with a hydrophobic surface. The results allow us to propose a better mask layer design dependent on wettability, reducing the risk of contamination from respiratory droplets.

Measuring Liquid-into-Liquid Diffusion Coefficients by Dissolving Microdroplet Electroanalysis

Diffusion is a fundamental process in various domains, such as pollution control, drug delivery, and isotope separation. Accurately measuring the diffusion coefficients (D) of one liquid into another often encounters challenges stemming from intermolecular interactions, precise observations at the liquid interface, convection, etc. Here, we present an innovative electrochemical methodology for determining the diffusion coefficient of a liquid into another liquid. The method involves precisely tracking the lifetime of a nonaqueous droplet. An organic droplet is placed on an ultramicroelectrode surrounded by an aqueous solution of potassium hexacyanoferrate(II/III). The droplet initially blocks the reduction or oxidation of the redox species. As the droplet dissolves, giving access to the conductive microelectrode surface, a continuously increasing current is observed in voltammetry and the amperometric i-t response. The electrochemical response thus directly reports on the flux of redox species on the electrode surface, allowing us to precisely determine the lifetime of the droplet. D values are directly determined through a combination of electrochemical analysis and the principles of droplet dissolution. We demonstrate the quantification of 1,2-dichloroethane and nitrobenzene into water, yielding diffusion coefficients of (11.3 ± 1.2) × 10-6 cm2/s and (5.2 ± 1.1) × 10-6 cm2/s, respectively. This work establishes a reliable electrochemical approach for quantifying diffusion coefficients based on droplet lifetime analysis.

Quantitation of phenylalanine and tyrosine from dried Blood/Plasma spots with impregnated stable isotope internal standards (SIIS) by FIA-SRM

BACKGROUND

Dried Blood Spot (DBS) analysis has been used for identification and quantification of diseases and disorders in large populations. Simply collecting blood or plasma samples on cotton paper, followed with an organic solvent extraction, many small molecules can be detected and quantified. In a typical procedure of DBS analysis in newborn screening, stable isotope internal standards (SIIS) are added to extraction solvent as a reference. However, this way of employing SIIS does not reflect extraction efficiency, or protein binding issues, nor does it reflect potential degradation that could occur. In addition, punched-out discs from larger DBS are known to have imprecision typically ≥ 15%.

METHODS

We developed and tested an approach, internal quantitative DBS (iqDBS), which delivers an exact volume of whole blood or plasma to a paper disc that is impregnated with a dried concentration of SIIS for quantitation. Amino acids were derivatized to make butyl esters and measured using Flow Injection Analysis with Selected Reaction Monitoring (FIA-SRM).

RESULTS

We demonstrated with phenylalanine and tyrosine improved sensitivity and accuracy by applying iqDBS.

CONCLUSIONS

We established a new method for quantitative analysis of small molecules from dried blood spots that incorporates stable isotope internal standard at the time of blood collection.

Porous Cellulose Thin Films as Sustainable and Effective Antimicrobial Surface Coatings

In the present work, we developed an effective antimicrobial surface film based on sustainable microfibrillated cellulose. The resulting porous cellulose thin film is barely noticeable to human eyes due to its submicrometer thickness, of which the surface coverage, porosity, and microstructure can be modulated by the formulations and the coating process. Using goniometers and a quartz crystal microbalance, we observed a threefold reduction in water contact angles and accelerated water evaporation kinetics on the cellulose film (more than 50% faster than that on a flat glass surface). The porous cellulose film exhibits a rapid inactivation effect against SARS-CoV-2 in 5 min, following deposition of virus-loaded droplets, and an exceptional ability to reduce contact transfer of liquid, e.g., respiratory droplets, to surfaces such as an artificial skin by 90% less than that from a planar glass substrate. It also shows excellent antimicrobial performance in inhibiting the growth of both Gram-negative and Gram-positive bacteria (Escherichia coli and Staphylococcus epidermidis) due to the intrinsic porosity and hydrophilicity. Additionally, the cellulose film shows nearly 100% resistance to scraping in dry conditions due to its strong affinity to the supporting substrate but with good removability once wetted with water, suggesting its practical suitability for daily use. Importantly, the coating can be formed on solid substrates readily by spraying, which requires solely a simple formulation of a plant-based cellulose material with no chemical additives, rendering it a scalable, affordable, and green solution as antimicrobial surface coating. Implementing such cellulose films could thus play a significant role in controlling future pan- and epidemics, particularly during the initial phase when suitable medical intervention needs to be developed and deployed.

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.

High density deposits of binary colloids

Colloids are essential materials for modern inkjet printing and coating technology. For printing and coating, it is desirable to have a high density of colloids with uniformity. Binary colloids, which consist of different size colloidal particles, have the potential to achieve high coating density and uniformity from size effects. We report a strategy to attain high-density deposits of binary colloids with uniform, crack-free, and symmetric deposits through droplet evaporation on micropillar arrays. We modify surfaces of micropillar arrays with plasma treatment to control their surface energy and investigate how binary colloidal fluids turn into well-controlled deposits during evaporation with X-ray microscopic and tomographic characterizations. We attribute temporary surface energy modification of micropillar arrays to the well-controlled high-density final deposits. This simple, low-cost, and scalable strategy would provide a viable way to get high-quality, high-density deposits of colloids for various applications.

Characterization of Sweat Drying Performance of Single Layered Thermal Protective Fabrics Used in High-Risk Sector Workers' Clothing

Absorption and transportation of moisture from sweat are the crucial properties of the fabrics used in performance clothing. Sweat moisture is a significant factor that may cause discomfort to the wearer. The majority of the injuries and fatalities that happen to the high-risk sector workers in their line of duty may be caused by inadequate comfort provided by the protective uniform. The purpose of this study is to scientifically investigate the sweat drying performance of the different protective fabrics used in high-risk sectors' workers' clothing. Firstly, this study experimentally analyzed the sweat drying of protective fabrics with different attributes under various ambient environments and wearers' internal physiology. Secondly, this study explained the phenomena of sweat drying in protective fabric through the theory of heat and mass transfer. Sweat drying performance of the fabrics used in functional clothing mainly depends on the evaporative resistance regardless of the presence of water and oil repellent coating on the fabric surface. The drying performance increases with the increased wetted area and increased air flow. The wetted area depends on the absorption and wicking properties of the fabrics. The findings of this research will advance the field by developing knowledge on sweat drying performance of fabrics used in protective clothing; in turn, this could provide better comfort and safety to high-risk sectors' workers.

4D study of liquid binder penetration dynamics in pharmaceutical powders using synchrotron X-ray micro computed tomography

The properties of pharmaceutical powders, and the liquid binder, directly influence the penetration behavior in the wet granulation process of the pharmaceutical industry. Conventional methods encounter challenges in understanding this fast process. In this work, an emerging synchrotron-based X-ray imaging technique (having fast imaging capability) was employed to investigate the internal process from 2D and 3D to real-time (in-situ with ms time intervals) 3D (also considered 4D) perspectives. Two commonly used excipients (lactose monohydrate (LMH) and microcrystalline cellulose (MCC)) were used to make binary mixtures with acetaminophen (APAP) as the active pharmaceutical ingredient (API). Isopropanol and water were employed as liquid binders in the single droplet impact method. Results showed that for most of the mixtures, the porosity increased at higher fractions of APAP. MCC mixtures experienced less agglomeration and more uniform pore distribution than LMH ones, resulting in a faster droplet penetration with isopropanol. Moreover, the imbibition-spreading studies showed that isopropanol penetration in MCC powders followed more unidirectional vertical movement than horizontal spreading. Our results also demonstrated that simultaneous granulation of LMH with water resulted in much slower penetration. This study revealed that synchrotron X-ray imaging can investigate 3D internal pore structures and how they affect the quantitively real-time internal penetration dynamics.