initiates and propagates a pandemic by combining models of droplet evaporation, aerodynamics and SIR 7 . Given the size distribution of respiratory droplets 8 , the airborne nuclei have a high probability of assimilation via oral or nasal passage. They might also deposit on objects of daily use to form fomites which can subsequently be assimilated by a person via touch. Although the infectivity of a given droplet-nuclei/fomite is linked to the initial viral load 9,10 as well its stability in different environments 11, 12, 13 , it is equally important to understand the desiccation and the precipitation dynamics of the infected droplet. The general practice is to study the viral activity in cellular environments 14 under diffusion effects 15, 16 where the precipitation dynamics are not very important. On the other hand, droplet embodies a plethora of fluidic transport 17, 18 and couples precipitation and evaporation of droplet to the agglomeration dynamics of the virions with the cellular material to which it is attached. Given the complexity of the experiment with actual viruses in respiratory fluid, such studies have been rarely attempted 20 . Mucosalivary fluids are known to have dissolved salts (~1 wt.%) in addition to mucus and enzymes 19, 20 . This letter uses dissolved NaCl in de-ionised water at 1 wt% as a simple surrogate liquid.
Inactive nanoparticles of polystyrene (mean size 100 nm) are added to this saline solution to emulate virus particle of similar size (CoV-2, Influenza) 21 Although having different mechanical and chemical properties, nanoparticles shed important insights into the transport and precipitation role of virions inside an airborne droplet. Viral loads occur in the range of 10 6 -10 9 per ml of the respiratory fluid 22 which translates into an approximate initial nanoparticles load of φnp ~ 0.0001 (in wt. % unless stated otherwise) in the given saline solution. However, precipitation dynamics at higher loads 23 presents a fundamental insight into nanoparticles interaction at high electrolyte concentration 24 as well as a crucial premise for several other applications 25 . To this end, φnp would also be varied from 0.01 to 0.1 for further investigation.
Given the experimental complexity associated with studying a mobile air-borne droplet, we have used an acoustic levitator to trap a droplet in the air (tec5) and allowed it to evaporate in a controlled ambience (T∞=28+0.2 °C, RH∞=41+2%). Acoustic levitation 26 has been extensively used to study the evaporation 27 and precipitation dynamics of a solute laden droplet 25, 28, 29, 30 . A droplet of the surrogate fluid having an initial diameter D0=550 μm+10 μm is inserted into one of the stable nodes of the acoustic levitator and imaged every 3 seconds at 30 frames per second (see S1 of Supporting Information) till the end of evaporation. The effective diameter of the droplet = √ 2 3 where dx and dy are the major and minor axis of the droplet, respectively. Figure 1b shows the lifetime of evaporation. The droplet monotonically reduces till the time instant t=tI where the shrinkage appears arrested. Subsequently, the shape of the droplet deviates from its initial sphericity (dx/dy=1) at t=tII and finally assumes its crystalline form at t=tIII shown for different concentrations of nanoparticles. The droplet shape evolves under evaporation and departs from its initial sphericity as shown by the plot of (dx/dy) at t=tII (Figure 1c ). The transition can be predicted as follows. The Peclet number for a levitated saline droplet 32 is = 0 ≈0.5 where Ds ~O(10 -9 ) m 2 /s is the diffusion coefficient of NaCl in water 33 .
Pe<1 indicates homogenous distribution of salt allowing the use of droplet volume to estimate its bulk concentration in the droplet. At a time t=teff corresponding to D/D0<0.26, the efflorescence limit (640 g/l) 34 is achieved within the bulk of the droplet. The close match between tII and teff is shown in Figure   1e proving Acoustic streaming around the droplet governs the internal flow field 26 and is visualized by adding 0.86 μm particles of latex (1.05 g/cc) at an initial concentration of 0.008 % wt. Illumination is done using a laser beam of 1 mm at 0.2 W (see S1 Supporting Information). The time averaged flow field in Figure   2a shows a circulatory motion within the droplet, where a fluid particle near its surface moves at a mean rate of 0.087+0.02 m/s. This homogenizes the salt molecules in the azimuthal direction (but not in the radial direction where it diffuses). The flow magnitude and direction agrees with previous studies of particle image velocimetry in evaporating levitated droplets and remains nearly constant throughout the droplet lifetime 36 as observed from Figures 2a(ii and iii) . Note that an ejected respiratory droplet is accompanied by a jet and subjected to atmospheric turbulence leading to similar rotatory motions 3 which is recreated in this case due to the acoustic streaming and torque provided by the levitator 26 .
Laser scatter in absence of 860 nm particles aid in visualizing the onset of precipitation. The scatter from the droplet is sampled at a rate of 50 fps (for details see S1; Supplementary ( Figure 2b (iv)) which shows an even higher uniformity in scatter. Although, spatial inception of efflorescence is difficult to identify, a drastic shape change could be observed when the bulk has crystallized as seen from the time lapse between Figure 2b (i-iv). The final cuboidal shape of NaCl 37 is observed from Figure 2b (v) at a time tIII=320~330 s. The shape evolution is better visualized using front illumination (see S1 of Supporting Information) as shown in Figure 2c . The spherical shape in Figure   2c Figure 3b (v).
Particles trapped near the upper layers of the precipitate are scanty and appear partially exposed (not stacked as multilayers) as shown in Figure 3b (v). In absence of particle loading, the surface topography of the precipitate is very smooth as shown in Figure 3b (iv). Thus, loaded particles/virions in levitated samples tend to be embedded mostly within the bulk. angle θ (~20° on glass and 60° on steel) 18 . Fluorescence images are acquired within the perimeter of the surface precipitate where the signal from the substrate corresponds to the exposed nanoparticles ( Fig.   4b (ii-iii), 4c(ii-iii)). The integrated intensity from the substrate is denoted as Isubstrate (Fig. 4b(ii) , 4c(ii)) while the same from the crystal is Ibulk (Fig. 4b(iii) , 4c(iii). The fraction of particles exposed on the surface is simply = ∑ where Itotal= Isubstrate +Ibulk. Since the given volume of droplet contains n~8 x10 6 particles, the average number of particles at the surface is nexp=nI. On glass, nexp~ 5x10 6 while in case of steel substrates, nexp~ 7x10 6 . The variation in the numbers between steel and glass is due to the affinity of the substrate-particle interaction 39 as well as the internal flow structure 40 . Nonetheless, precipitates in fomites indeed show a greater percentage of exposed particles (~ 80-90%) on the substrates as compared to embedded in crystals as in air-borne counterparts. The Probabilistic Analysis for National Threats Hazards and Risks (PANTHR) database 41 predicts the virus lifetime to be significantly shorter (~100 times) in air-borne precipitates when compared to those on solid surfaces.
This correlates to the presented experimental findings that the virions are more exposed in dried settled droplets as opposed to their airborne counterparts.
In summary, a nanocolloidal system is successfully used to mimic the evaporation and precipitation dynamics of an isolated mucosalivary droplet. Theoretical and experimental arguments are presented to show how the evaporation leads to salt crystallization which traps the virion-substitutes at different layers of the air-borne precipitate. Fluorescent microscopy correlates the lower survival rates of virus in the air-borne precipitates to its lower number of exposed virions.