Simultaneous imaging of multi-pore sweat dynamics and evaporation rate measurement using wind tunnel ventilated capsule with infrared window

Abstract

Sweat evaporation is critical to human thermoregulation, but current understanding of the process on 20 μm to 2 cm scale is limited. To this end, we introduce a wind-tunnel-shaped ventilated capsule with an infrared window for simultaneous infrared sweat imaging and evaporation rate measurement. Implementing the capsule in pilot human subject tests suggests that the common assumption of sweat being an isothermal film is only valid when the evaporation rate is low and sweat forms puddles on the skin. Before transitioning to this filmwise mode, sweating occurs in cyclic dropwise mode, displaying a 3x higher mass transfer coefficient in the same conditions. Imaging highlighted distinct phenomena occurring during and between these modes including out-of-duct evaporation, pulsating droplets, temporary and eventually lasting crevice filling, and individual drop-to-film spreading. In all, sweat evaporation is an impactful area that our results show is ripe for exploration, which can be achieved quantitatively using the introduced platform.

Keywords: biophysics; devices.

Graphical abstract

Figure 1

Overview of the wind tunnel ventilated capsule with infrared transparent window (A) Comparison of velocity distributions in a central vertical plane and streamlines within traditional cylindrical and the wind tunnel ventilated capsules at air flow rate of 0.5 L⋅min⁻¹. (B) Annotated image showing the arrangement of the wind tunnel capsule for simultaneous measurement of the sweat evaporation rate and multi-pore scale imaging on a passively heated subject’s forehead. (C) An example of through-sapphire window mid-wave infrared image of sweat droplets evaporating from the skin surface.

Figure 2

Characterization of air flow distribution within the wind tunnel capsule and benchmarking of evaporation rate measurements employing the device (A) Air velocity distribution on a central plane in a wind tunnel ventilated capsule with two acrylic windows with air flow rate of 0.3 L⋅min⁻¹ visualized using particle image velocimetry (PIV). (B) Comparison of PIV-measured and simulated x-averaged [according to axis definitions in (A)] y-velocity profiles for air flow rates of 0.3, 0.6, and 1 L⋅min⁻¹. (C) An example of experimental measurement of an artificial 0.64 cm² square water film evaporation rate when the film is allowed to evaporate and when it is replenished to maintain equilibrium via a syringe pump; inset shows the schematic of the experimental setup. (D) Impact of humidity probe distance from evaporation area quantified using 0.64 cm² square water film experiments and simulations at 0.5 and 1 L⋅min⁻¹ air flow rate (to facilitate comparison, the ratio of average probe to average outlet water vapor concentrations is plotted); the insets show the evaporation sections of capsules with different probe locations (diffuser not attached), the perforated probe tip, and water vapor concentration profiles at 1 L⋅min⁻¹ and two probe locations.

Figure 3

From the onset of cyclic dropwise to established filmwise sweating (A) The sweat evaporation mass flux measured with 0.1 L⋅min⁻¹ air flow rate and per the 1.93 cm² evaporation area and (B) corresponding absolute and relative wet surface area (i.e., covered by sweat) obtained from analyzing MWIR imaged surface sweat dynamics including (C) onset and (D) established cyclic dropwise sweating and (E) transition to (F) the filmwise mode.

Figure 4

Droplet characteristics during cyclic dropwise sweating (A) The number of active pores (droplets) vs. time within the 1.93 cm² evaporation area (the red line shows 1-min moving average). (B) The wet area vs. number of active pores (the red line shows a linear fit to the data whose equation is displayed). (C) Diameter vs. time plot showing examples of a long-lasting and large droplet (orange line) as well as multiple sequential (i.e., from same pore) brief and small droplets (blue line). (D) Maximum diameter vs. duration of the droplets. (E and F) Probability and cumulative distribution function (CDF) histograms of (E) the maximum diameter and (F) droplet duration.

Figure 5

Forced drying out of filmwise sweating through increased air flow rate The time series of (A) the flow rate (blue line) and water vapor concentration (orange line) measured by the probe (C_probe) for the 1.93 cm² evaporation area and corresponding (B) evaporation mass flux and (C) six MWIR images corresponding to time points (i–vi) indicated in (B).

Figure 6

Synthesis of the evaporation rate measurement and MWIR image analysis (A) Evaporation mass flux vs. wet area (or wet area fraction) under constant air flow rate (0.1 L⋅min⁻¹); results for simulated and measured evaporation from square water films with a varied area are also shown. (B) The forced "dry out" experiments: the quasi-steady state evaporation mass flux vs. air flow rate; simulations and measured evaporation of isothermal square water films at 34°C and 0.64 cm² are also shown (additional simulation for 36.5°C and area of 1 cm² are also included). (C) Schematic summary of the directly observed or implied pore or multi-pore scale processes underlying sweat evaporation including out-of-duct evaporation, cyclic dropwise evaporation, temporary crevice filling, drop-to-film spreading through stratum corneum (SC) hydration, pore bridging via crevice, and film puddles with non-uniform exterior temperature.