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Bio-Heat and Mass Transfer

Influence of Substrate Nature on the Evaporation of a Sessile Drop of Blood

[+] Author and Article Information
David Brutin1

Benjamin Sobac

 Aix-Marseille University, IUSTI UMR 7343 CNRS, Marseilles 13013, France

Céline Nicloux

 Institut de Recherche Criminelle de la Gendarmerie Nationale, IRCGN-DCIH-DATO, Rosny sous Bois 93110, France

1

Corresponding author.

J. Heat Transfer 134(6), 061101 (May 08, 2012) (7 pages) doi:10.1115/1.4006033 History: Received March 21, 2011; Revised December 06, 2011; Published May 08, 2012; Online May 08, 2012

We fully characterize the natural evaporation of human drops of blood from substrates and substrate-dependent behavior. The heat flux adsorbed by the drops for evaporation is measured by means of a heat flux meter. A side-view measurement enables access to the drop contact angle, wetting diameter, and initial height. A top-view camera allows for the monitoring of the drying regime (deposition, gelation, and fracturation). This directly measured heat flux is related to the evaporative mass flux obtained from the mass of the drop, and the two show good agreement. Both types of measurements indicate that regardless of the substrate type, there is first a linearly decreasing regime of evaporation when the drop is mostly liquid and a second regime characterized by a sharp decrease. We show that the evaporation dynamics are influenced by the substrate’s wettability but not by the substrate’s thermal diffusivity. The different regimes of evaporation exhibited by glass and metallic substrates are explained in terms of evaporation fluxes at the drop surface. In the case of wetting drops (below 40 deg), the evaporation flux is very important along the drop periphery and decreases across the interface, whereas in the case of nonwetting drops (about 90 deg), the evaporation flux is almost uniform across the droplet’s surface. We show that these different evaporation fluxes strongly influence the drying behavior. In the case of metallic substrates, this enables the formation of a uniform "glassy skin" around the droplet surface and, in the case of glass substrates, the formation a skin along the drop periphery with an inward gelation front. This behavior is analyzed in terms of the competition between the drying time and the gel formation time. Unstable drop surfaces were observed at high initial contact angles and are very similar to those of polymer drops.

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Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Drops of blood on different substrate types

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Figure 2

Schematic of the experimental setup used for heat transfer measurements

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Figure 3

Initial and final drop shapes on glass (initial drop mass: 5.67 mg, initial drop diameter: 4.79 mm, initial drop height: 0.41 mm, initial drop contact angle: 20.5 deg)

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Figure 4

Heat and mass transfer during blood drop evaporation on glass: heat flux, evaporative mass flow rate, and image at specific time steps (initial drop mass: 5.67 mg, initial drop diameter: 4.79 mm, initial drop height: 0.41 mm, initial drop contact angle: 20.5 deg, drop interface sequence from 0 to 12 min in steps of 2 min)

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Figure 5

Initial and final drop shapes on gold (initial drop mass: 9.53 mg, initial drop diameter: 3.45 mm, initial drop height: 1.53 mm, initial drop contact angle: 91.9 deg, drop interface sequence from 0 to 36 min in steps of 6 min)

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Figure 6

Heat and mass transfer during the blood drop evaporation on gold: heat flux, evaporative mass flow rate, and image at specific time steps (initial drop mass: 9.53 mg, initial drop diameter: 3.45 mm, initial drop height: 1.53 mm, initial drop contact angle: 91.9 deg)

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Figure 7

Initial and final drop shapes on aluminum (initial drop mass: 9.51 mg, initial drop diameter: 3.39 mm, initial drop height: 1.66 mm, initial drop contact angle: 95.7 deg, drop interface sequence from 0 to 36 min in steps of 6 min)

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Figure 8

Heat and mass transfer during blood drop evaporation on aluminum: heat flux, evaporative mass flow rate and image at specific time steps (initial drop mass: 9.51 mg, initial drop diameter: 3.39 mm, initial drop height: 1.66 mm, initial drop contact angle: 95.7 deg)

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Figure 9

Comparison with experiments by Pauchard and Alain [10] performed using a polymer drop; the two dried drops had almost the same contact angle and evaporated at the same room humidity. The initial diameter was also almost the same.

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