Precipitation dynamics of surrogate respiratory sessile droplets leading to possible fomites

Abstract

Hypothesis

The droplets ejected from an infected host during expiratory events can get deposited as fomites on everyday use surfaces. Recognizing that these fomites can be a secondary route for disease transmission, exploring the deposition pattern of such sessile respiratory droplets on daily-use substrates thus becomes crucial.

Experiments

The used surrogate respiratory fluid is composed of a water-based salt-protein solution, and its precipitation dynamics is studied on four different substrates (glass, ceramic, steel, and PET). For tracking the final deposition of viruses in these droplets, 100 nm virus emulating particles (VEP) are used and their distribution in dried-out patterns is identified using fluorescence and SEM imaging techniques.

Findings

The final precipitation pattern and VEP deposition strongly depend on the interfacial transport processes, edge evaporation, and crystallization dynamics. A constant contact radius mode of evaporation with a mixture of capillary and Marangoni flows results in spatio-temporally varying edge deposits. Dendritic and cruciform-shaped crystals are majorly seen in all substrates except on steel, where regular cubical crystals are formed. The VEP deposition is higher near the three-phase contact line and crystal surfaces. The results showed the role of interfacial processes in determining the initiation of fomite-type infection pathways in the context of COVID-19.

Introduction

The ongoing COVID-19 pandemic has disrupted global travel, healthcare systems, social interactions, and business activities worldwide. Primary transmission of the virus occurs at the microscale level, where respiratory droplets rapidly spread the SARS-CoV-2 amongst human beings [1], [2]. To arrest the transmission of the virus, wearing a facemask and maintaining social distances has been advised by the scientific and medical community worldwide [3], [4], [5]. The ejected droplets are in size range of 1–2000 $\mu$m [6] and creates two possible scenarios of infection. Smaller droplets can evaporate, precipitate [7], travel far [8], and stay airborne for a sufficiently long time before being directly inspired by another healthy human being [9]. On the other hand, the larger droplets may settle under gravity or impinge on a material surface, forming fomites [10], [11]. In either scenario, infection mechanics, which involve virus survivability [12], remains elusive. In this article, we shall limit our discussion to the physicochemical transformations within a VEP (virus emulating particles) [3], [7], [13] loaded surrogate respiratory droplet drying on different commonly available real life surfaces.

The pattern formation during the evaporation of suspended particles in a sessile droplet was first explained by Deegan et al.[14]. The capillary flow inside the droplet led to the deposition of the solute particles near the pinned contact line. The pinning of the contact line was attributed to the inherent roughness of the substrate. Ring deposition morphs to dome shape for moderate to high particle concentration (5–20 $\mathit{mg}/\mathit{mL}$) mainly due to inter-particle interactions [15]. The pattern was independent of hydrophobicity at lower particle concentrations ($<$5 $\mathit{mg}/\mathit{mL}$) for which ring deposition was observed [15]. Further, Shmuylovich et al. [16] and Maheswari et al. [17] observed the formation of concentric-rings pattern during pinning and de-pinning of the contact line, while a single monolith pattern was observed by Baldwin et al. [18]. The thick-edge deposit near the contact line can be altered by surface patterning [19], [20], [21], [22], choice of particles shape [23], solvent-vapor-substrate interaction [24], convection currents [25], surface roughness and addition of other soluble components [26]. The flow patterns inside the evaporating droplet influence the final deposition of the particles. The evaporation flux in a sessile droplet was triggered by diffusion of solvent at the liquid–air interface, and it diverged near the contact line of a pinned droplet [27], [28]. The divergent evaporative-flux and pinning of the droplet contact line led to the capillary flow towards the droplet contact line [14], [29].

In contrast to the capillary flow, Marangoni flow is observed due to surface tension changes at the liquid–air interface. This surface tension difference can arise due to temperature changes, i.e., thermocapillary flow [30] or solute concentration changes, i.e., solutal Marangoni flow [31], [32], [33]. The thermo-capillary flow was created either by the temperature difference between the liquid–air interface and underlying substrate [34] or due to the non-uniform cooling (due to non-uniform evaporation) along with the liquid–air interface [35], [36]. Barmi et al. [37] numerically modeled the evaporation rate and internal circulation in a thermo-capillary flow of a sessile droplet with a pinned contact line. The thermal gradient at the air–liquid interface was shown to be the driving force for the flow, and the flow magnitude decreases during the later stages of evaporation due to the reduction in a thermal gradient. Solutal Marangoni flows were prompted by the surface tension difference between the edge and the apex of the evaporating droplet, along the liquid–air interface [38], [31]. The surface tension difference arises due to non-uniform evaporation at the liquid–air interface, which increases salt concentration near the droplet contact line. The Marangoni-induced flow can be directed to or away from the contact line depending on the solution constituents[31], [32]. The Marangoni circulation was generally observed during the evaporation of sessile saline-droplets [39], [37], [40], [41], [42], [43]. Shahidzadeh et al. [41] found that the deposition stains of saline droplets were different from colloidal droplets. The formation of salt crystal stains depended on wetting properties, nucleation pathways, and growth rates. On a super-hydrophobic substrate, Shin et al. [40] showed the formation of ‘‘ring” or ‘‘igloo” shaped salt depositions depending on the initial salt concentrations and evaporation rates. The resulting precipitate shapes depended on the combined effect of solubility, evaporation rate, and contact line hysteresis. The flow pattern inside the evaporating salt droplet was experimentally shown by Kuznetsov et al. [44] and Efstratiou et al. [43]. Efstratiou et al. [43] showed a uniform outward flow towards the contact line during most of the evaporation stage. However, during the crystallization stage, a jet flow towards the nucleation site was observed. A detailed overview of a drying droplet on a substrate was provided by Larson et al. [45] and Parsa et al. [46].

Evaporation of bio-fluid droplets has been applied as a medical diagnosis tool in earlier works [47], [48], [49]. Devineau et al. [50] analyzed the pattern formation of evaporating protein droplets containing suspended polystyrene particles. The suppression of the coffee ring effect due to protein adsorption on the surface of the particles was observed. In another study [51], the formation of edge deposition or uniform deposition of particles was shown to be dependent on the protein charge. The bio-fluid compounds usually contain dissolved salt in them, and the deposition patterns of these salt-protein biofluid droplets were reported [52], [53], [54]. The formation of dendritic patterns was shown by Darwich et al. [55] for a bi-component solution of oppositely charged alginate polysaccharides and gold nanoparticles. The dendritic structure formation was dependent on the salt concentration, drying mode, and particle size. Choudhury et al. [56] studied the drying of colloidal gel containing dissolved salt and showed that the final deposition morphology depended on the type of host gel, salt concentration, and substrate used. Carreon et al. [57] discussed the effectiveness of first-order statistics (FOS) and grey level-co-occurrence matrix (GLCM) methods to characterize the protein deposition patterns and their complex texture. Pathak et al. [54] studied the precipitation dynamics of two components (salt and proteins) bio-fluids at different constituent compositions. The final deposition patterns (either crystalline or dendritic) were distinctive and dependent on the initial ratio of the two components.

The above discussion provides an overview of the evaporation of sessile droplets containing suspended particles, salts, and biological compounds. However, respiratory fluid is complex and demands a more critical approach to study. Sefiane et al. [58] investigated the pattern formation from drying droplets of various fluids, including biofluid. Researchers have shown that complex bio-fluid evaporation involving blood [59] and the synovial fluid [60] agrees well with observations and theoretical models of simple fluid droplets. Mukhopadhyay et al. [61] showed recently different pattern formation driven by interfacial energy on drying blood droplets. Vejerano et al. [62] have presented a detailed experimental exposition of respiratory droplets (surrogate and real), drying on a surface, and their impact on virus survivability. However, several questions remain. What is the role of the substrate beyond wettability and thermal conductivity? What does the flow inside a droplet of complex fluid look like? If virus particles are dispersed inside such a droplet, how do they aggregate in the final residue? We shall discuss experimental findings that are not intuitively apparent and present a heuristic understanding in this article.

The details of the materials and methods used in the present work are given in Section 2. Section 3 provides the results and discussion of this paper. Here we discuss the variation in evaporation lifetime of a surrogate droplet resting on these substrates with different thermal conductivity and wettability. The internal flow dynamics are discussed next. We establish the link between the onset of crystallization and the dynamics of evaporation and flow. We discuss the qualitative as well as quantitative features of the crystallization pattern. Further, we explain how the crystallization pattern distributes the VEP within the precipitate. Finally, the summary and conclusions from the present work are shown in Section 4.