Insights into the mechanics of pure and bacteria-laden sessile whole blood droplet evaporation

TL;DR

This work deciphers the mechanics of sessile whole-blood drop evaporation for pure and bacteria-laden samples, revealing a universal three-stage process tied to sol-gel transitions: (A) rapid edge gelation, (B) inward-propagating gelation front forming a wet gel, and (C) slow drying with lamination-delamination and crack formation. A axisymmetric lubrication model couples evaporation flux, height, and RBC concentration, producing quantitative height profiles and precipitate thickness that agree with optical profilometry. Across biologically relevant bacterial concentrations ($c\le 10^9$ CFU/mL), evaporation rates and final residue patterns remain largely invariant, while extremely high levels ($c\sim 10^{12}$ CFU/mL) alter crack morphologies in the corona. The results, supported by SEM, profilometry, and confocal imaging, provide a mechanistic framework for interpreting dried blood residues relevant to diagnostics and for understanding how infection-related changes in blood composition influence desiccation patterns.

Abstract

We study the mechanics of evaporation and precipitate formation in pure and bacteria-laden sessile whole blood droplets in the context of disease diagnostics. Using experimental and theoretical analysis, we show evaporation process has three stages based on evaporation rate. In the first stage, edge evaporation results in a gelated contact line along the periphery through sol-gel phase transition. The intermediate stage consists of gelated front propagating radially inwards due to capillary flow and droplet height regression in pinned mode, forming a wet-gel phase. We unearthed that the gelation of the entire droplet occurs in the second stage, and the wet-gel formed contains trace amount of water. In the final slowest stage, wet-gel transforms into dry-gel, leading to desiccation-induced stress forming diverse crack patterns in the precipitate. Slow evaporation in the final stage is quantitatively measured using evaporation of trace water and associated transient delamination of the precipitate. Using axisymmetric lubrication approximation, we compute the transient droplet height profile and the erythrocytes concentration for the first two stages of evaporation. We show that the precipitate thickness profile computed from the theoretical analysis conforms to the optical profilometry measurements. We show that the drop evaporation rate and final dried residue pattern do not change appreciably within the parameter variation of the bacterial concentration typically found in bacterial infection of living organisms. However, at exceedingly high bacterial concentrations, the cracks formed in the coronal region deviate from the typical radial cracks found in lower concentrations.

Figures (16)

• Figure 1: Schematic of the experimental set up and sample top, side and bottom view images of dried blood drop precipitate. Scale bar depicted in white represents $1$ mm.
• Figure 2: (a) Schematic of the coordinate system, composition of the blood and initial condition of sessile blood drop evaporation. (b) Non dimensional volume ($V/V_0$) regression plotted as a function of non dimensional time ($t/t_*$) for whole blood + EDTA, whole blood + EDTA + $10^6$ CFU/ml KP (Klebsiella pneumoniae) bacteria and whole blood + EDTA + $10^9$ CFU/ml KP bacteria respectively. The different stages of sessile blood drop evaporation depicted as A, B and C respectively. The solid black straight line denotes the linear evaporation regime from which the true regression curves deviate at the end of stage A. (c) Top view, side view and bottom view time sequence images of the evaporation process respectively. Scale bar for the top, side and bottom view represents $1$ mm respectively. The timestamps are in non dimensional units ($t/t_*$). (d) Top view depicting the final precipitate at $t/t_*=1$ for EDTA, $10^{6}$ CFU/ml, $10^{9}$ CFU/ml, and $10^{12}$ CFU/ml respectively. The scale bar denotes $1$ mm respectively.
• Figure 3: Schematic representation of the various processes occurring in stage A of blood droplet evaporation. (a) Initial configuration of the sessile blood droplet at $t/t^*=0$ depicting the evaporative flux and the radial outward capillary flow inside the evaporating droplet. (b,c,d) Magnified view of the outer edge of the droplet depicting the precursor film and the outward transport of RBCs towards the edge at (b) $t/t^*=0$, (c) $t/t^*=0.1$, and (d) $t/t^*=0.2$ respectively. The blue vertical line shows the gelation front propagating radially inwards. (e) A small control volume (CV) near the three phase contact line depicting the sol phase in which RBCs are present inside the CV and getting transported across the surface of the CV. (f) Wet gel phase in the CV as RBCs concentration increases.
• Figure 4: (a) Top, side and bottom view time sequence images of stage A at different non dimensional time instants $t/t_*=0,{\:}0.05,{\:}0.1,{\:}0.15,{\:}0.2$ respectively. Scale bar for top, side and bottom view represents 1.2 mm, 1.3 mm and 1 mm respectively. (b) Schematic representing the initial configuration of the evaporating droplet ($t/t_*=0$). (c) Schematic representing the evaporating droplet at the end of evaporation stage A ($t/t_*=0.2$) (d,e) Non dimensional geometrical drop parameters $G(t)$ (normalized contact angle (${\theta}/{\theta}_0$), normalized contact radius ($R/R_0$), normalized drop height ($h/h_0$), and normalized gelation radius ($r_g/r_{g0}$)) evolution as a function of non dimensional time $t/t_*$.
• Figure 5: (a) Top, side and bottom view time sequence images of stage B at various non dimensional time instants $t/t_*=0.2,{\:}0.3,{\:}0.4,{\:}0.5,{\:}0.6,{\:}0.67$ respectively. The Scale bar represents 1 mm. (b) Non dimensional geometrical drop parameters $G(t)$ (normalized contact angle (${\theta}/{\theta}_0$), normalized drop height (${h}/h_0$), normalized contact radius ($R/R_0$) and normalized gelation radius ${r_g}/{r_{g0}}$) evolution as a function of non dimensional time $t/t_*$. (c) Schematic representing the evaporating droplet at the beginning of stage B ($t/t_*=0.2$) and end of stage B ($t/t_*{\sim}0.67-0.7$) respectively.