A micro-to-macroscale and multi-method investigation of human sweating dynamics

Ankit Joshi; Cibin T. Jose; Kambiz Sadeghi; Konrad Rykaczewski; Shri H. Viswanathan; Sincere K. Nash; Stavros A. Kavouras

REVIEW 4 major objections 5 minor 4 references

A micro-to-macroscale and multi-method investigation of human sweating dynamics

T0 review · 4 major / 5 minor · reviewed 2026-08-15 · deepseek-v4-flash

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Pith's one-line read Sweating on most skin proceeds through three microscale modes, from porewise cycling to a thin film, and the paper shows that macroscale sweat measurements follow from these imaged dynamics.

desk verdict Careful, image-rich observational study with a genuinely new meniscus observation and a useful capsule-design caution, but the macro-micro temporal story rests on an unverified co-location assumption. read the letter →

arxiv 2505.04789 v1 pith:RIWAZKA7 submitted 2025-05-07 physics.bio-ph

Cibin T. Jose , Ankit Joshi , Shri H. Viswanathan , Sincere K. Nash , Kambiz Sadeghi , Stavros A. Kavouras , Konrad Rykaczewski This is my paper · ORCID

classification physics.bio-ph

keywords sweatproductionevaporationdropletdynamicsmidwaveinfraredimagingopticalcoherencetomographygalvanicskinconductancestratumcorneumhydrationsaltdeposits

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper sets out to show that visible sweating on non-glabrous skin is not a steady oozing of hemispherical droplets but a staged process: sweat first reaches and evaporates at individual pores in cycles, then spreads into shallow puddles, and finally merges into a thin film. By imaging the forehead with midwave infrared and optical coherence tomography while simultaneously measuring sweat rate, skin conductance, and stratum corneum hydration, the authors argue that the rise times and plateaus of the macroscopic signals line up with these microscale modes. If true, this changes how sweat onset, skin hydration, and sensor-skin contact are understood in thermoregulation, cosmetics, textiles, and wearable devices. It also implies that salts left behind after sweat dries reshape the next round of sweating, making it spread faster as a film.

What carries the argument

The carrying mechanism is the coupled measurement stack: midwave infrared (MWIR, 3–5 µm) thermography, optical coherence tomography (OCT) cross-sectional imaging, and visible macrophotography, run alongside ventilated-capsule sweat rate, galvanic skin conductance, and dielectric stratum corneum hydration. MWIR sees thin water at individual pores, OCT resolves the cross-sectional shape of menisci, puddles, and films, GSC tracks sweat entering ducts before it reaches the surface, HYD tracks hydration of the outer roughly 50 µm of skin, and SR tracks evaporative flux. The mode sequence (cyclic porewise, transition, filmwise) is the organizing identity that links the microscopic images to the macroscopic curves.

What would settle it

Place the conductance electrodes and the ventilated capsule over the exact forehead patch that the MWIR camera images, or image a second patch adjacent to the sensors, and time pore activation pore-by-pore. If sweat first appears at the sensor-side pores more than about 1.7 minutes before or after the imaged patch, then the claimed ordering of GSC before MWIR before SR and HYD is a site-mismatch artifact rather than a physiological sequence.

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Extended reading notes

Core claim

The paper's central claim is that thermal sweating on non-glabrous skin passes through three imaged modes—cyclic porewise, transition with spreading shallow puddles, and filmwise—and that the area-averaged physiological signals (galvanic skin conductance, sweat rate, and stratum corneum hydration) are explained by these modes. In the porewise mode, sweat periodically fills and evaporates at individual pores; OCT shows the liquid-air interface as a shallow convex or flat meniscus, not the hemispherical droplet of traditional textbook pictures. The transition mode consists of irregular puddles roughly 0.1 to 1 mm deep with contact angles of 20 to 40 degrees, and the filmwise mode is a thin film tens to hundreds of micrometers thick. GSC rises about 1.7 ± 0.8 minutes before MWIR-visible pore activity because it senses sweat filling the ducts, while sweat rate and hydration rise after onset as the number of active pores and local stratum corneum hydration grow. During a second heating after drying, salts deposited from evaporated sweat wick the new sweat into a rapidly spreading film, making mode shifts faster and more variable across subjects.

Load-bearing premise

The load-bearing premise is that the patch of forehead being imaged and the nearby skin under the conductance electrodes and the sweat-rate capsule are sweating in the same state at the same times; if sweat onset is spatially patchy across the few centimeters separating them, the claimed ordering of signals collapses.

Editorial extensions

If this is right

The textbook image of sweat as hemispherical droplets sitting on non-glabrous skin should be replaced by shallow menisci at pores, shallow puddles during transition, and thin films at high sweat rates.
Interpreting GSC and HYD as evidence of uniform stratum corneum hydration from beneath, before sweat reaches the surface, is not supported; hydration begins locally and cyclically at active pores.
Ventilated capsules that use impinging air jets can locally cool the skin and under-report sweat rate, so capsule flow geometry matters for absolute sweat rate values.
After a drying episode, salt residues on skin make subsequent sweat spread as a film rather than puddles, shortening and making more variable the time spent in porewise and transition modes.
Microscopic hairs on the forehead can wick sweat away from pores and participate in evaporation during all modes.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

A controlled follow-up comparing second-heating sweat spreading on the same subject with and without gently wiping the skin between cycles would isolate the salt-deposit mechanism from the confounding effect of already-elevated core temperature.
Because the cyclic porewise mode is driven by sympathetic nerve oscillations, MWIR-visible cycling frequency could become a non-invasive, spatially resolved proxy for sudomotor nerve activity in psychophysiology studies.
The finding that local cooling changes sweat rate implies that any ventilated capsule perturbs the quantity it measures; reporting a true local sweat rate may require extrapolation to zero cooling or concurrent skin-temperature measurement.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

Short version: this is a careful observational study with two genuinely useful results—the first OCT imaging of thermal sweating on non-glabrous skin, and a clear demonstration that air-jet ventilated capsules locally suppress sweat rate by cooling. The descriptive staging of porewise → transition → filmwise modes is well supported by the images, and the observation that sweat forms shallow menisci rather than hemispherical droplets is a real challenge to a common textbook picture. The paper is worth refereeing, but the macro-micro temporal story has a site-mismatch problem that needs to be addressed.

The new material: OCT cross-sections of forehead sweating from onset to film, showing pore-confined sweat as flat or slightly convex menisci, then shallow puddles (contact angles around 20–40°), then thin films. The timing data (GSC leads MWIR onset by ~1.7 min; SR and HYD lag) and the salt-deposit interpretation of second-heating dynamics are more speculative, though the authors do flag that the salt deposits were not directly imaged. The air-jet capsule result is a clean, practically important negative: too much local cooling gives artificially low sweat rates.

The soft spots, in order of severity. First and most important: the GSC electrodes, the ventilated capsule, and the HYD probe are not on the imaged patch. They sit 2–4 cm away, under conditions that are locally perturbing—dry airflow in the capsule, isotonic gel under the electrodes, probe pressure. The paper uses MWIR onset in the central patch as time zero for all instruments and then interprets the ordering (GSC first, SR/HYD after) as a read on duct filling and stratum corneum hydration. That inference assumes the lateral sites sweat on the same schedule as the imaged site, and that the capsule itself does not delay local onset. The paper demonstrates exactly this delay for the air-jet capsule, and the cylindrical capsule also imposes dry airflow. The stress-test note lands; this is a load-bearing assumption, not a minor detail. Second, n=6, one skin site, supine posture, visually classified modes. For an observational study that is acceptable, but it limits the quantitative claims. Third, the salt mechanism is plausible but inferred; the authors say so, but the Discussion leans on it harder than the data warrant. Fourth, the movies are only 'available upon request' rather than archived. Minor: the air-jet pilot is n=1.

Who is this for? People working on sweat sensing, skin biophysics, thermal comfort, and wearable devices. It is a hypothesis-generating characterization, not a confirmed model. I would send it to review, with the co-location issue front and center. A revision that either co-locates macro and micro measurements on a smaller area, or explicitly softens the temporal-ordering claims, would make the paper solid. If the authors cannot co-locate, they should present the macro-micro alignment as suggestive rather than explanatory.

Referee Report

4 major / 5 minor

Summary. This paper reports a multi-method study of human forehead sweating in six supine subjects undergoing passive heating, cooling, and reheating. The authors combine macroscopic measurements (ventilated-capsule sweat rate, galvanic skin conductance, and dielectric stratum corneum hydration) with three microscale imaging modalities (visible-light macro photography, midwave infrared, and optical coherence tomography). They describe a cyclic porewise mode, a transition mode with shallow spreading puddles, and a filmwise mode, and interpret the temporal relationships among SR, GSC, and HYD in terms of these imaged dynamics. They also compare a novel air-jet ventilated capsule with a standard cylindrical capsule and report that the former over-cools the skin and depresses measured sweat rate. The paper further proposes that salts deposited during the first heating stage alter the mode of sweating in the second heating stage, and that sweat at pores forms shallow menisci rather than the hemispherical droplets often drawn in the literature.

Significance. If the main claims hold, the paper offers a valuable advance in linking microscale sweat dynamics to macroscopic physiological signals. The simultaneous application of SR, GSC, HYD, MWIR, and OCT on non-glabrous skin is original and technically demanding; the air-jet capsule comparison is a useful practical result; and the reported staging of porewise, transition, and filmwise modes, with supporting images and movies, is internally consistent and generally agrees with prior literature. The authors are also candid about several limitations, including the inability to resolve salt deposits and the exploratory nature of the fingertip comparison. However, the central temporal claim, that GSC rises before and SR/HYD rise after MWIR-visible pore sweating, depends on an assumption of synchronized onset across a spatially distributed instrument layout that the manuscript does not validate, and the salt-driven mechanism is asserted rather than directly supported. These issues are load-bearing and require attention.

major comments (4)

[Sections 2.1, 3.3, Table 1, SM S1.3] The temporal comparisons between macroscopic signals and imaged pore dynamics assume that the GSC electrodes, the ventilated capsule, and the HYD probe observe the same sweating state as the central MWIR/OCT patch. As described in Section 2.1 and Figure 1b, the GSC electrodes and capsule are attached to the sides of the forehead, and SM S1.3 places the HYD probe 2 to 4 cm from the imaged center. Table 1 then sets the MWIR onset to 10 minutes for all instruments. This is exactly the equivalence that is at issue. The ventilated capsule imposes forced dry airflow that locally cools and dries the skin (the paper itself demonstrates the stronger air-jet version depresses SR in Figure 2d), and the GSC electrodes sit under isotonic gel, which hydrates and occludes the skin. Without a co-location check or other evidence that onset and mode timing are synchronized across the roughly 2- to 4-cm instrumented area, the reported ordering (GSC rises 1.7 +/- 0.8 min before MWIR onset; SR and HYD rise 1.4 +/- 0.7 and 2.7 +/- 1.8 min afterward) is not established. This assumption is load-bearing for the paper's central explanation and should be addressed by co-locating measurements, by reanalyzing the timing with an explicit synchronization check, or by substantially softening the causal interpretation.
[Section 4.2, Figure 8, Conclusions point 5] The salt-deposition mechanism is presented as a key factor explaining the faster filmwise second-stage sweating, but the authors state that the salt deposits were 'undetectable using any of the employed imaging techniques'. This is an acknowledged missing support. The observed differences between first and second heating stages are also consistent with elevated core temperature, residual stratum corneum hydration, or residual sweat under the GSC electrodes, which the authors mention only as contributing to faster onset. As written, point 5 of the Conclusions asserts a mechanistic claim that the paper's own evidence cannot distinguish. The salt-facilitated film-spreading mechanism should be explicitly framed as a hypothesis, and, if retained as a conclusion, should be supported by direct salt visualization or by a control condition such as washing the skin between heating stages.
[Section 3.3, Table 1] The mode onset times and the claimed correspondences between mode transitions and inflections in SR, GSC, and HYD rely on visual classification of MWIR and OCT images. No quantitative criterion is given for distinguishing cyclic porewise, transition, and filmwise modes, and no inter-rater reliability or sensitivity analysis is reported. Because the alignment of macroscopic inflections with mode transitions is part of the central narrative, a reproducible rule (for example, based on wet-area fraction, MWIR contrast, or OCT-derived film thickness) or at least a sensitivity check on the visual thresholds should be provided.
[Section 3.3] The mean lead/lag values (GSC -1.7 +/- 0.8 min, SR +1.4 +/- 0.7 min, HYD +2.7 +/- 1.8 min) are presented without statistical tests. With n = 6 for the first stage and n = 5 for the second stage, the paper should report the full distribution of individual differences and, ideally, a paired comparison or sign test to establish that the ordering is consistent across subjects rather than driven by one or two outliers. As it stands, the mean values and standard deviations do not by themselves support the claim that GSC systematically precedes MWIR-visible pore sweating.

minor comments (5)

[SM S1.3] The text says the hydration meter probes the outer '~50 mm' of skin; this should be '~50 µm'. The acquisition rate is also given as '2,000 kHz', which is presumably '2 kHz'.
[Section 3.2 and Figure 8 caption] There are minor typographical errors: 'human trails' should be 'human trials', and 'separted' in the Figure 8 caption should be 'separated'.
[Table 1] Table 1 is difficult to parse because the first- and second-stage columns are not visually separated and several entries are missing (for example, subject 5's F-mode row and some GSC values). Please reformat the table with clear stage headers and mark missing data explicitly.
[Figures 3 and 5] The captions state that GSC and HYD measurements are scaled 'as indicated in legend', but the specific scaling factors are not stated in the main text or captions. Please provide the scaling factors so that the reader can interpret the plotted magnitudes.
[Data Availability] The data availability statement says that MWIR and OCT movies 'can be provided upon request'. Given the reproducibility value of the imaging data, please consider depositing the processed movies and a data summary in a permanent repository.

Circularity Check

0 steps flagged · score 2.0 of 10

No circular derivation: imaging and physiological measurements are independent observations; self-citations are contextual, not load-bearing.

full rationale

This is an observational multi-method study rather than a derivation or prediction exercise. No model parameters are fitted to the reported data, and no quantity called a prediction is obtained by construction from an input. The central assertions—cyclic porewise/transition/filmwise modes, GSC leading MWIR-visible pore sweating, SR/HYD lagging, and salt-affected second-stage spreading—are supported by co-registered MWIR, OCT, and macroscopic time traces presented in Figures 3–5 and Table 1. The paper invokes the authors' prior work [36] for the mode vocabulary, MWIR sensitivity to thin water films, the experimental setup, and a prior dropwise/filmwise mass-transfer coefficient result, but none of these citations is used to force the new measurements: the modes are re-observed directly in the current MWIR/OCT data, and the mass-transfer coefficient is not an input to any equation in this paper. The explanations of mode timing via stratum corneum hydration and salt deposits are explicitly interpretive ('likely', 'can likely be explained') rather than derived from the cited constants, so they are not circular. The main validity threat—that GSC, HYD, and SR are measured on skin displaced 2–4 cm from the imaged patch and under local perturbation (occlusive gel, ventilated airflow, 1.5–2 N probe force)—is a spatial-synchronization/correctness concern, not a case where an output is equivalent to an input by definition. Under the hard rules, that concern belongs to correctness risk, not to circularity scoring.

Assumptions & free parameters 0 free parameters · 5 assumptions · 0 invented entities

No free parameters are fitted; the study is observational. The axioms are the measurement assumptions behind each instrument and the assumed spatial uniformity of forehead sweating. No new physical entities are introduced; salt deposits are a known skin feature inferred rather than newly postulated.

assumptions (5)

domain assumption MWIR temperature contrast is a valid proxy for the presence of liquid sweat on the skin.
The imaging method relies on evaporative cooling and the 3 um water absorption band to identify wet regions; used throughout Sections 3.2-3.4 and Figure 3.
domain assumption The ventilated-capsule sweat rate is a local measure of sweat that reaches and evaporates at the skin surface and reflects secretion under the capsule.
Standard premise of the century-old method, cited in refs [2,18,37,38]; used to compare capsules and to set baselines in Figures 2-5.
domain assumption Sweating dynamics are synchronized across the instrumented forehead area, so that camera measurement on the central forehead can be time-compared with GSC and capsule measurements on the sides.
GSC electrodes and the ventilated capsule are placed laterally while imaging is central (Figure 1b and SM S1.3). Timing claims such as 'GSC increases 1.7 +/- 0.8 min before MWIR onset' assume spatial uniformity of onset.
domain assumption The Delfin MoistureMeter output (AU) monotonically reflects stratum corneum hydration in the outer tens of micrometers.
Instrument premise from ref [34]; cited in SM S1.3. The main text's '50 mm' is a typo for microns.
domain assumption Passive heating and cooling via the water perfused garment changes thermal drive in a controlled, reproducible manner across subjects and stages.
Protocol based on refs [36,37,38]; used to compare first and second heating stages in Section 3.3.

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Cite this review

Pith. "Pith review of A micro-to-macroscale and multi-method investigation of human sweating dynamics." pith.science (2026). https://pith.science/paper/RIWAZKA7

@misc{pith2026250504789,
  author       = {Pith},
  title        = {Pith review of: A micro-to-macroscale and multi-method investigation of human sweating dynamics},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/RIWAZKA7}},
  note         = {Machine review of arXiv:2505.04789}
}

read the original abstract

Sweat secretion and evaporation from the skin dictate the human ability to thermoregulate and thermal comfort in hot environments and impact skin interactions with cosmetics, textiles, and wearable electronics or sensors. However, sweating has mostly been investigated using macroscopic physiological methods, leaving micro-to-macroscale sweating dynamics unexplored. We explore these processes by employing a coupled microscale imaging and transport measurement approach used in engineering studies of phase change processes. Specifically, we employed a comprehensive set of macroscale physiological measurements (ventilated capsule sweat rate, galvanic skin conductance, and dielectric epidermis hydration) complemented by three microscale imaging techniques (visible light, midwave infrared, and optical coherence tomography imaging). Inspired by industrial jet cooling devices, we also explore an air jet (vs. cylindrical) capsule for measuring sweat rate. To enable near simultaneous application of these methods, we studied forehead sweating dynamics of six supine subjects undergoing passive heating, cooling, and secondary heating. The relative dynamics of the physiological measurements agree with prior observations and can be explained using imaged microscale sweating dynamics. This comprehensive study provides new insights into the biophysical dynamics of sweating onset and following cyclic porewise, transition, and filmwise sweating modes, and highlights the roles of stratum corneum hydration, salt deposits, and microscale hair.

Figures

Figures reproduced from arXiv: 2505.04789 by the authors.

**Figure 1.** Image showing (a) overview of the experimental setup including imaging methods (MWIR and OCT/macrophotography lenses are cyclically placed above the imaged area) and (b) of instrumented subject forehead during the first sweating stage. 2. Methods 2.1 Overview We developed a custom experimental setup to enable simultaneous measurements using GSC, HYD, and SR and periodically altered MWIR and OCT/macrophotography (see… view at source ↗

**Figure 2.** (a)-(b) whole and cross-sectioned three-dimensional models of (a) the cylindrical and (b) air jet capsules; (c) the evaporation flux as a function of dry air flow rate measured for different ventilated capsules placed on artificial sweating surface (1 cm2 square and heated water thin film at 34ºC), and (d) sweat rates (SR) for subject 7 comparing two cylindrical and U-bend air jet capsules with air flow rate of 0.5 … view at source ↗

**Figure 3.** Representative sweating and drying dynamics during subject 2 heating, cooling, and second heating stages: (a) the sweat rate (SR), galvanic skin conductance (GSC), dielectric epidermis hydration (HYD) plotted against time along with annotations about microscopic sweating modes observed through imaging (note: the GSC and HYD measurements are scaled as indicated in legend to facilitate comparison); the corresponding (… view at source ↗

Figures from the paper (5 more)

**Figure 4.** Figure 4: The MWIR, macrophotography, and OCT images corresponding to the second heating stage of the trial shown in [PITH_FULL_IMAGE:figures/full_fig_p013_4.png]

**Figure 5.** Figure 5: (a) the sweat rate (SR), (b) the galvanic skin conductance (∆GSC), (c) dielectric epidermis hydration (HYD) plotted against time along with annotations about microscopic sweating modes observed through imaging (note: the baselines for ∆GSC for first and second heating …

**Figure 6.** Figure 6: a shows several hairs within puddles of sweat during the late transition mode, with some standing up while others stick to the skin surface. The sequence illustrates how two hairs are forced to bridge when two puddles coalesce. Using OCT, we often imaged hair sticking …

**Figure 7.** Figure 7: The relative variation of macroscopic measurements with the three sweating modes onsets indicated plotted along with corresponding data from Gerrett et al. [15] (a) the sweat rate (SR) vs. the galvanic skin conductance (∆GSC), (b) SR vs. dielectric epidermis hydration …

**Figure 8.** Figure 8: Schematics contrasting the difference that the salt depositions induces between sweating during the (a-c) first and (e-f) second heating stages, that are separted by sweat evaporation and salt deposition (d). 5. Conclusions We conducted an integrative and multi-scale s…

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Reference graph

Works this paper leans on

4 extracted references · 4 canonical work pages

[1]

2018 Sweat from gland to skin surface: production, transport, and skin absorption

Gerrett N, Griggs K, Redortier B, Voelcker T, Kondo N, Havenith G. 2018 Sweat from gland to skin surface: production, transport, and skin absorption. J Appl Physiol 125, 459–469

work page 2018
[2]

2024 Simultaneous imaging of multi-pore sweat dynamics and evaporation rate measurement using wind tunnel ventilated capsule with infrared window

Jaiswal AK et al. 2024 Simultaneous imaging of multi-pore sweat dynamics and evaporation rate measurement using wind tunnel ventilated capsule with infrared window. iScience 27, 110304

work page 2024
[3]

2004 Measurement of hydration in the stratum corneum with the MoistureMeter and comparison with the Corneometer

Alanen E, Nuutinen J, Nicklén K, Lahtinen T, Mönkkönen J. 2004 Measurement of hydration in the stratum corneum with the MoistureMeter and comparison with the Corneometer. Skin Research and Technology 10, 32–37

work page 2004
[4]

2020 Rational design of sun and wind shaded evaporative cooling vests for enhanced personal cooling in hot and dry climates

Rykaczewski K. 2020 Rational design of sun and wind shaded evaporative cooling vests for enhanced personal cooling in hot and dry climates. Appl Therm Eng , 115122

work page 2020

Pith tools

Reviewed August 15, 2026 · model on record in the stance chip above.

[1] [1]

2018 Sweat from gland to skin surface: production, transport, and skin absorption

Gerrett N, Griggs K, Redortier B, Voelcker T, Kondo N, Havenith G. 2018 Sweat from gland to skin surface: production, transport, and skin absorption. J Appl Physiol 125, 459–469

work page 2018

[2] [2]

2024 Simultaneous imaging of multi-pore sweat dynamics and evaporation rate measurement using wind tunnel ventilated capsule with infrared window

Jaiswal AK et al. 2024 Simultaneous imaging of multi-pore sweat dynamics and evaporation rate measurement using wind tunnel ventilated capsule with infrared window. iScience 27, 110304

work page 2024

[3] [3]

2004 Measurement of hydration in the stratum corneum with the MoistureMeter and comparison with the Corneometer

Alanen E, Nuutinen J, Nicklén K, Lahtinen T, Mönkkönen J. 2004 Measurement of hydration in the stratum corneum with the MoistureMeter and comparison with the Corneometer. Skin Research and Technology 10, 32–37

work page 2004

[4] [4]

2020 Rational design of sun and wind shaded evaporative cooling vests for enhanced personal cooling in hot and dry climates

Rykaczewski K. 2020 Rational design of sun and wind shaded evaporative cooling vests for enhanced personal cooling in hot and dry climates. Appl Therm Eng , 115122

work page 2020