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. 2024 Mar 27;58(15):6532–6539. doi: 10.1021/acs.est.4c00850

Heat Transfer by Sweat Droplet Evaporation

Mohadese Beigtan †, Marta Gonçalves †,‡, Byung Mook Weon †,‡,*
PMCID: PMC11025549  PMID: 38538556

Abstract

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Sweating regulates the body temperature in extreme environments or during exercise. Here, we investigate the evaporative heat transfer of a sweat droplet at the microscale to unveil how the evaporation complexity of a sweat droplet would affect the body’s ability to cool under specific environmental conditions. Our findings reveal that, depending on the relative humidity and temperature levels, sweat droplets experience imperfect evaporation dynamics, whereas water droplets evaporate perfectly at equivalent ambient conditions. At low humidity, the sweat droplet fully evaporates and leaves a solid deposit, while at high humidity, the droplet never reaches a solid deposit and maintains a liquid phase residue for both low and high temperatures. This unprecedented evaporation mechanism of a sweat droplet is attributed to the intricate physicochemical properties of sweat as a biofluid. We suppose that the sweat residue deposited on the surface by evaporation is continuously absorbing the surrounding moisture. This route leads to reduced evaporative heat transfer, increased heat index, and potential impairment of the body’s thermoregulation capacity. The insights into the evaporative heat transfer dynamics at the microscale would help us to improve the knowledge of the body’s natural cooling mechanism with practical applications in healthcare, materials science, and sports science.

Keywords: sweat droplet, imperfect evaporation, evaporative cooling, heat index, biofluids

Short abstract

Studying sweat evaporation is crucial to understanding how the human body manages heat under different conditions. This research explores sweat droplet dynamics and their impact on thermoregulation, which is vital for assessing heat-related risks in hot and humid environments.

Introduction

Sweating is essential to the body’s thermoregulation system, as it helps in dissipating heat by evaporation from the skin’s surface.1,2 However, this system can be disrupted in several everyday scenarios, such as extreme physical activity during sports, firefighting, and mining, as well as spending time in a hot sauna or bath or even during a typical day in a heat wave season. Global warming, more recently referred to as “global boiling”, has increased the occurrence and intensity of heat waves. This leads to prolonged periods of abnormally high temperatures, often coupled with high humidity levels resulting in aggravated heat-related health risks.3−11 Physiological examinations conducted under various environmental conditions suggest that the apparent temperature of the human body rises in humid weather, making it feel hotter than the actual temperature, as determined by the heat index.12 This can be hazardous to the body, even at lower temperatures, when the ability to counter heat stress is uncertain. The heat index was established in the late 1970s and was derived through a series of equations determined by extensive biometeorological research that examines the combined effect of temperature and humidity on perceived temperature.13,14 This index evolved into an environmental health metric, enabling the identification of critical conditions that could significantly risk human thermoregulation.12,15 Although physiological examinations can define thermal safety for the human body, exploring the microscopic scale of cooling phenomena is essential to understanding the underlying mechanism. Notably, the microscopic dynamics of evaporative cooling, a physical phenomenon occurring on the surface of human skin, still need to be better understood despite their importance. Interestingly, skin temperatures exhibited a significant correlation with humidity perception. This could be attributed to ineffective sweat evaporation on the skin surface, which led to an impaired ability to regulate thermoregulation.16 The evaporation of sweat droplets has gained considerable attention in several areas, such as health monitoring,17−20 smart clothing,21−24 and microelectronics cooling.25,26 Sweat droplets are critical for dissipating heat from the skin’s surface through evaporation; therefore, we have a better understanding of their microscopic dynamics and related phenomena mediated by evaporation.

Sweat is a complex biofluid that contains solvents, solutes, and biomolecules. It carries significant physiological information.27−30 It is an ultradilute yet nutrient-rich fluid consisting of 98% water and 2% solutes that include various amino acids, metabolites, and inorganic components such as salts.31 The evaporation and deposition dynamics of biofluid droplets exhibit a high level of complexity.32−34 The complexity inherent in biofluid droplets alters their evaporation dynamics by manipulating their hydrodynamic and thermodynamic properties. Research on simpler systems, such as salt or salt–protein droplets, has indicated that varying the salt ratio or protein percentage results in diverse crystalline morphologies, which can indicate different evaporation dynamics.35−40 The presence of salts and certain biocomponents can affect a droplet’s ability to evaporate entirely, implying that these solutes may possess hygroscopic properties that result in the sorption of water vapor from the surrounding environment when the air is unsaturated.36 A rise in water salinity diminishes the evaporation rate due to decreased water vapor pressure at the droplet’s surface. Furthermore, the process of crystallization significantly impacts droplet evaporation by altering the concentration of solutes inside the droplet, thereby affecting the rate of water vapor transfer from the droplet to the atmosphere.41 In a humid or warm environment, condensation of water vapor leads to the formation of breath figures (BF), manifesting as tiny droplets on a surface. This phenomenon typically occurs when a surface colder than the surrounding air is exposed to moisture or humid air. A salty droplet functions as a humidity sink, causing the saturation vapor pressure of the droplet to vary during growth due to moisture sorption.42 Various biomolecules and inorganic components in biofluids may enhance the complexity of the dynamics of evaporation and deposition through potential interactions.43 The behavior of biofluid evaporation and deposition is significantly influenced by its components and their interactions, posing limitations on prediction and difficulties in controlling related application systems. We expect sweat droplets, which are complex biofluids, to exhibit intricate dynamics during evaporation.

Our study uses an artificial sweat droplet and an artificial skin model to investigate the intricate dynamics of sweat evaporation at a microscopic scale under varying environmental conditions. The combined effect of different temperatures and humidities is evaluated by determining their impact on the evaporative cooling of a sweat droplet undergoing evaporation. We observed imperfect evaporation of sweat droplets due to their complex composition, while perfect evaporation of water droplets occurred under different environmental conditions. In this work, we define imperfect evaporation as when the droplet evaporation does not end with a solid deposit remaining on the surface; however, it leaves a liquid or an equilibrium solid–liquid phase. Interestingly, we show how microscopic moisture sorption on unclean skin (a skin surface where sweat deposits persist due to evaporated sweat droplets) can be the reason for imperfect evaporation in sweat droplets under conditions of high relative humidity and minimize evaporative heat transfer, which shows up at a higher heat index. Through this research, we aim to gain a better understanding of sweat droplet evaporation dynamics and how they affect the heat index, which may have significant implications in the field of human thermoregulation.

Results and Discussion

Sweat Imperfect Evaporation

The “feels-like” temperature, or the heat index, an indicator of heat risk and comfort, is empirically determined by the interplay of relative humidity and air temperature. The primary focus of this study is to examine how sweat evaporation dynamics at a microscopic scale, the most critical mechanism to cool the body, affect the heat index. We note that a designed chamber was used to conduct all of the experiments under controlled environmental conditions. Additionally, to keep the current model as similar as possible to a real scenario, we conducted most experiments on an artificial skin substrate with wettability identical to human skin. Complementary experiments were carried out on a glass substrate for imaging purposes. To gain insights into the complexity of sweat droplet evaporation, we compared the evaporation behavior of sweat and water droplets under identical environmental conditions of relative humidity ≈85% (shown in Figure 1) and ≈15% (shown in Figure S1) at ≈25 °C temperature. The droplets were initially 5 μL in volume. We used side-view optical microscopy in Figure 1, Supporting Information, Figure S1, Movies S1, and S2. Our findings reveal that the sweat droplet experienced imperfect evaporation, while the water droplet simply underwent complete evaporation (the left and right side droplets in Figure 1A, respectively), showing complex evaporation dynamics of sweat droplets. Figure 1B illustrates multistage evaporation dynamics of the contact angle (θ) and radius (r) for both sweat and water droplets. In particular, the third stage of evaporation for the sweat droplet reveals a distinctive feature of imperfect evaporation, where a tiny sweat droplet remains with constant θ and r over time, indicating that evaporation is hindered at high humidity. Figure S1 shows similar dynamics between sweat and water droplets; however, their total evaporation time is significantly reduced, and the third stage existing in Figure 1 is absent under low humidity conditions. In high humidity, the evaporation dynamics are markedly different between sweat and water droplets, even though their compositions are nearly identical except for about 2% variation in biomolecules and inorganic components in the sweat droplet. We explore the reasons behind the different evaporation patterns of sweat droplets. Sweat is composed of insoluble and soluble components in water, which can affect the hydrodynamics and thermodynamics of droplet evaporation. Adding biomolecules and inorganic components to the water can change the physical properties of the solution, resulting in different hydrodynamics within the droplet. Thus, we compare the physical properties of sweat and water, including the initial contact angle, surface tension, and viscosity (Figure 1C). We observed similar physical properties between the two fluids and expected similar hydrodynamics. Therefore, the complexity of sweat droplet evaporation may be rooted in its kinetic or thermodynamic properties.

Figure 1.

Figure 1

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Imperfect evaporation of sweat droplets. (A) Side-view images taken during the evaporation of droplets on an artificial skin substrate in an environment with 85% relative humidity and a temperature of 25 °C depict imperfect evaporation of sweat droplets, while water droplets evaporate entirely. (B) Dynamics of the sweat and water droplet contact angle (θ) and radius (r) during evaporation reveal the occurrence of imperfect evaporation through a third stage with constant θ and r for the sweat droplet. (C) Physical properties of artificial sweat and water include the initial contact angle, surface tension, and viscosity. The error bars are the standard deviation for an average of five measurements.

Effect of Humidity and Temperature on Imperfect Sweat Evaporation

To understand the underlying mechanism of imperfect sweat evaporation and the effect of different environmental conditions on it, we investigated the effect of temperature and humidity on the evaporation dynamics of sweat droplets. The complete videos of the sweat droplet evaporation (Movies S3–S10) were taken using inverted optical microscopy in the controlled chamber under different humidities of 15, 35, 55, and 75% at two distinct temperatures: room temperature (≈25 °C) and high temperature (≈40 °C). The representative frames are displayed in Figure 2, where the time is normalized to the final evaporation time (tf). We note that for conditions with imperfect evaporation, the tf is set when the droplet volume and shape do not show significant changes over time. In this experimental setting, a glass substrate was used to visualize the final deposits of sweat droplets in different environmental conditions. We characterize the initial contact angles of water and sweat droplets on the glass substrate (Figure S2). Due to the difference in wettability compared to human skin, an artificial skin substrate was utilized instead of glass for quantifying evaporation dynamics and heat transfer of sweat droplets. For room-temperature conditions (Figure 2A), we observe that for lower humidities (15 and 35%), there is complete evaporation of the sweat droplet, leading to the formation of a solid deposit in the final stage. However, in higher humidities (55 and 75%), the final stage is represented by a liquid or an equilibrium liquid–solid droplet that remains constant without reaching a fully solid deposit (showing the so-called imperfect evaporation in this work). For high-temperature conditions (Figure 2B), we observe that all droplets are completely evaporated except 75%, resulting in a partial liquid deposit at the end. As temperature increases, the kinetic energy of the droplet molecules increases, leading to a higher evaporation rate and, consequentially, a higher evaporation-driven capillary flow. The uniqueness of the final deposit morphology can depend on the solution composition and the surrounding conditions while evaporation occurs. The present results show a preferential formation of dendritic crystal deposits at lower temperatures and small cubic crystals at higher temperatures. The dendrite formation was verified on artificial skin after sweat droplet evaporation, similarly to the glass substrate, by using X-ray computed tomography to overcome visualization difficulties (Figure S3). A phase diagram summarizing the effect of humidity and temperature on imperfect evaporation through the final deposit patterns is shown in Figure S4. Moreover, a quantified analysis of the droplet’s radius dynamics is provided in Figure S5, which corresponds to the image frames displayed in Figure 2. These results demonstrate that the temperature and humidity conditions determine the final deposit phase composition and the contact line evolution during evaporation. In Figures 2 and S5, black arrows highlight the droplets depinning moments. It is noticeable that the droplet depinning occurs in the earlier stages at 40 °C (Figure 2B) compared to 25 °C (Figure 2A). This behavior is described for sessile droplets evaporating at higher temperatures because of the faster increment of the force acting on the triple contact line.44 Regarding the humidity effect, the droplet depinning occurs in the earlier stages for higher humidities at both 40 °C (Figure 2B) and 25 °C (Figure 2A). We assume that this behavior is related to a long-lasting ending stage of sweat droplet evaporation under high humidity conditions. Due to that elongated stage, the depinning apparently happens earlier when normalizing the time to tf. However, the depinning occurs later in the real-time scale due to a slower evaporation rate at higher humidity.

Figure 2.

Figure 2

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Imperfect sweat evaporation at different temperatures and humidities. (A) Sweat droplet evaporation on a glass substrate at a fixed operation temperature (To) of 25 °C and (B) 40 °C with a ranging humidity of 15–75%; the sweat droplets and the final deposits are shown during evaporation. The visualized dynamics of sweat evaporation reveal that imperfect evaporation depends on an interplay between temperature and humidity. Depending on the temperature, there is no complete evaporation at higher humidities, and the final deposit is liquid or partially liquid. The red dashed frames indicate the conditions where imperfect evaporation was present.

Sweat imperfect evaporation is significant under high humidity conditions. The experimental observations demonstrate the effect of temperature and humidity on the final deposit of sweat droplets. Furthermore, since the evaporative cooling rate can be a limiting factor for the body’s ability to regulate and adapt to extreme conditions, it becomes crucial to also quantitatively define the sweat droplets’ lifetime under different environmental conditions to provide a physical meaning to the heat index.16,45 The tf dependence on the temperature and humidity was analyzed by measuring the sweat droplet mass change with time on an artificial skin substrate. The droplets’ initial volume is 5 μL for all the conditions with a ranging humidity of 15, 35, 55, and 75% and ranging temperatures of 25, 30, 35, and 40 °C. The complete dataset is presented in Supporting Information, Figure S6. The higher the humidity level, the longer it takes for the evaporation of the sweat droplet to reach an undetectable mass. The most distinct behavior comes for the RH ≈ 75% condition, which is discussed previously in Figure 2. Here, the increment in tf is until the undetected mass is of ≈1000 s, double from the other conditions, resulting in a tf = 3677 s. A complementary analysis was carried out by analyzing the power law scaling of m2/3 versus t (Supporting Information, Figure S7), where the plotted data linearity indicates that the evaporation mechanism is governed by diffusion-limited evaporation, and a deviation from it would indicate that other perturbations are affecting the evaporation dynamics of a sweat sessile droplet.43,46 We visualize that the nonlinearity of the evaporation dynamics is significantly noticeable at a high humidity of RH ≈ 75%. This nonlinearity could be attributed to the everlasting presence of the liquid residue, which is unable to evaporate completely at high humidities. To understand the interaction between temperature and humidity in the final evaporation time, the acquired tf for all conditions is compared (Supporting Information, Figure S7 panel C), correlating the sweat droplet lifetime with the potential risks associated with prolonged exposure to extreme environmental conditions. Our results indicate that when the humidity rises, the temperature effect on the tf aggravates and plays a bigger role. For a high humidity of RH ≈ 75%, both high and low temperatures correspond to higher evaporation times, which could hinder the immediate evaporative cooling necessity of the body. These results highlight the necessity of considering both temperature and humidity synergy when estimating the consequences of the evaporation time of a sweat droplet in the human body. As described above, different evaporation dynamics are notorious for varying the humidity, highlighting the importance of unveiling the basis for the imperfect evaporation of sweat droplets, which could be rooted in the sweat components at the final stages and the deposit formation ability.

Reversibility in Sweat Evaporation

Here, we define imperfect evaporation as when the evaporation flux is almost zero or a reverse flow acts simultaneously, resulting in a liquid phase staying on the surface for a time scale much longer than the typical evaporation time scale (see Figure S8). We checked the possible reversibility for sweat droplet evaporation depending on the relative humidity. First, we let a sweat droplet evaporate in a low relative humidity of 20%, leaving behind a solid dendritic-like deposit on the surface. Then, we gradually increase the humidity level to ≈80% and in situ monitor the sweat deposit behavior (Figure 3, Movie S11). Snapshots in Figure 3C show that the sweat solid deposit starts to sorb the surrounding moisture at ≈50% relative humidity and above (t4), resulting in a tiny droplet forming on the surface as the chamber’s relative humidity increases (t7). These results show that the sweat imperfect evaporation is reversible. The moisture sorption by sweat deposits can be caused by (I) hygroscopic and deliquescence properties of some sweat components and (II) possible moisture condensation in high humidity. Even though 98% of the sweat is water, about 2% is of different biocomponents like Urea and Lactate and electrolytes like Na, K, Ca, and Cl, which have hygroscopic and deliquescence properties.47−49 Hygroscopy refers to the ability of a substance to attract and hold moisture from the surrounding air due to its physical properties, such as its surface area and intermolecular forces. This can cause the substance to become damp. Deliquescence is a more extreme form of hygroscopicity, where a substance absorbs so much moisture from the environment that it dissolves in the absorbed water. In other words, deliquescence is when a solid substance absorbs enough moisture to form a liquid solution. This occurs when the vapor pressure of the absorbed moisture is higher than that of the solid, which causes the solid to dissolve. Deliquescence takes place at the deliquescence relative humidity (DRH).50 While this process and its temperature dependence are well-understood for inorganic salts, they have not been thoroughly explored for biofluid droplets such as sweat, which comprise a mixture of diverse organic and inorganic materials. In addition, increasing humidity and the presence of impurities may change the dew point of the sweat droplet, leading to the possibility of condensation at room temperature.42

Figure 3.

Figure 3

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Evaporation–deposition reversibility. (A) Schematic illustration of perfect and imperfect evaporation at low and high relative humidities. (B) Plot of the chamber’s relative humidity change over time. (C) Corresponding snapshot images of sweat deposits on the glass substrate show the moisture sorption. At the same time, the chamber’s relative humidity is increased according to the relative humidity plot in panel (B) and the evaporation while the chamber’s relative humidity is decreased. The black colored t indicates the period when the sweat droplet deposit is in a solid phase. The red colored t indicates the period when the sweat droplet deposit is partially or completely in a liquid phase. The asterisk represents the frames where evaporation is occurring, indicating reversibility.

Evaporative cooling on the skin surface depends on the collective evaporation of many sweat droplets since our skin generates sweat droplets continuously until it regulates the body’s temperature. The evaporation of several sweat droplets on our skin increases the humidity locally, which may affect the evaporation of sweat droplets. Therefore, a multiple sweat droplet configuration was created to investigate the evaporation and its reversibility in a more realistic scenario (Supporting Information, Figure S9 and Movie S12). First, we let a single sweat droplet evaporate at 20% relative humidity at room temperature and form a solid dendrite deposit on the surface. Then, several droplets of different sizes were placed surrounding the solid deposit at other time stamps to simulate the dynamic of sweating. Interestingly, we found moisture sorption by the solid deposit even at a low humidity of 20% as we placed new droplets. All of the droplets evaporate after stopping the placement of new droplets, leaving behind solid deposits. In multiple sessile droplet evaporation, neighboring droplets interact via their vapor fields, which results in a spatially non-uniform “shielding” effect.51−53 The so-called “shielding” effect describes that the presence of vapor from other droplets reduces the evaporation rate (and hence increases the lifetime) of a droplet in comparison to a single sweat droplet evaporation system. Here, the vapor fields generated by each droplet evaporation increase the local humidity around the solid deposit, resulting in moisture sorption even at 20% relative humidity. Since the droplet scale is microscopic compared to the experimental environment (chamber), the vapor generated by the evaporation of multiple droplets (the local humidity) does not change the environmental relative humidity shown by the sensors (20%). This finding shows how unexpectedly unclean skin (a skin surface where sweat deposits remain on it from evaporated sweat droplets) can disturb sweat evaporation, even under safe environmental conditions. The moisture sorption by unclean skin in low humidity at room temperature possibly causes harm to the human body in unexpected situations as not only the evaporative cooling would be absent but also there would be the risk of heat sorption.

Imperfect Sweat Evaporation and the Heat Index

Now, we try to understand how imperfect evaporation of sweat droplets can elaborate on the body’s ability to thermoregulate in different environmental conditions. We employ infrared (IR) thermal imaging to check the evaporative heat transfer of sweat droplets during evaporation under different environmental conditions on the artificial skin substrate. IR images of sweat droplets during evaporation and their evaporative cooling graphs for different relative humidities (25, 35, 55, and 75%) at temperatures of 27, 35, and 42 °C are shown in Figure 4 and Supporting Information, Figures S10 and S11, respectively.

Figure 4.

Figure 4

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Linking heat transfer and heat index by sweat droplet evaporation on artificial skin. (A) IR thermal images of sweat droplets during evaporation in ranging humidity of 25–75% at 27 °C operating temperature (To). (B) Extracted evaporative cooling of sweat droplets for different humidities at 27 °C. (C) For different temperatures, the evaporative heat transfer versus relative humidity (inset presents the corresponding coolest points versus relative humidity). (D) Calculated heat index and evaporative heat transfer relationship as a function of humidity for different temperatures. The background color gradient indicates the transition between safe (yellow) and dangerous (red) according to the heat index criteria reported by the NWS.

The IR images obtained during the droplet evaporation process revealed significant cooling characteristics at 25% relative humidity, whereas negligible cooling features at 75% (Figure 4A). The images demonstrated that once the droplet was placed on the substrate (t = 0), the cooling process commenced. After complete evaporation, the substrate reached an ambient temperature of approximately 27 °C. To quantify the evaporative cooling (ΔT) of the sweat droplets (Figure 4B), we calculated the temperature difference between the coolest point within the droplet zone (Tcool) and the temperature of the substrate outside the droplet zone (To) during evaporation (ΔT = To – Tcool), driven from the corresponding IR images. We observe a more substantial evaporative cooling effect in lower humidity levels, with a temperature reduction of approximately 8 °C for 25% relative humidity, compared to only 2 °C for 75% relative humidity. Figure 4B illustrates three stages of evaporative cooling behavior: (I) at the beginning of sweat evaporation (t < 6 min), the droplet temperature experiences a rapid decrease due to evaporative cooling. (II) As evaporation progresses, the evaporative cooling gradually diminishes. (III) In the final stage, after the complete evaporation of the droplet, the local temperature reaches an equilibrium with the ambient temperature (acclimatization).

Stage (I) exhibits the highest level of evaporative cooling across all humidity levels. The gradual decline in evaporative cooling during stage II can be attributed to the loss of droplet volume through evaporation, as smaller droplets are more quickly influenced by the surrounding temperature. In higher humidity conditions, the rate of evaporative cooling decrement is lower due to a slower evaporation rate and slower loss in droplet volume. In stage (III), after complete evaporation, the temperature aligns with the ambient temperature, indicating a thermal equilibrium. Interestingly, we observed an additional stage (IV) in the evaporative cooling, specifically at high humidity levels, such as 55 and 75%. In this stage, a residual droplet effect can be observed, characterized by a nonzero evaporative cooling fluctuating at the end caused by the remaining tiny droplet on the surface of the substrate.

The evaporative heat transfers (Q) in Figure 4C are estimated based on the corresponding evaporative cooling data (ΔT) in Figure 4B derived from the corresponding IR images. We chose the representative data of ΔT at 3 min after evaporation started for RH = 25, 35, 55, and 75%. Then, we derive the relationship of ΔT as a function of RH based on the experimental data and mathematical fitting (all the fittings own a value of R2 > 0.9700). Finally, we calculate the heat transfer of a sweat droplet. The heat loss by evaporation of a single sweat droplet is formulated as Q = mcΔT (Q is taken at t = 3 min, m is the mass of the droplet, and c is the specific heat for water, a reasonable approach since sweat only differs by 2% in solute concentration to water). By doing so, we found the equation Q as a function of RH, which allows us to extrapolate heat transfer values for different humidities. This data processing is applied to all the experimental data sets from three different temperatures. We also note that in the case of total heat transfer in Figure 4B, we verified that the integration of the evaporative cooling data results in approximately the same value range; however, the total time for the heat loss to occur in high humidity (75%) is around three times longer than that for low humidity (25%). Figure 4C shows evaporative heat transfer decreases by relative humidity for three different temperatures, reaching minimal values in high humidities, which can disturb our body’s ability to cool down through sweat evaporation. It is notable to mention that even though the evaporative heat transfer is increased for higher temperatures at an identical RH due to the higher evaporation flux, the amount of transferred heat is not large enough to cool down to a sufficiently low temperature since the droplet’s coolest point is over 30 °C (look at the inset in Figure 4C). Interestingly, the green dashed line shows that the same evaporative heat transfer value exists for sweat droplet evaporation at 27 °C and 55% with 42 °C and 75%, confirming the interplay of both temperature and humidity to determine the cooling ability through sweat evaporation as we also visualize in Figure 2. We directly show the relationship between the heat index (which in this work is calculated based on the previous literature14) and the evaporative heat transfer for our results in varying relative humidities at three different temperatures (Figure 4D). Our findings indicate that elevated humidity levels reduce evaporative cooling and heat transfer, yielding an elevated heat index for all temperatures. All the heat index lines show an increment in the slope for the smaller evaporative heat transfers, owning a higher relative humidity level and more possibilities of imperfect evaporation. Our results obtained from microscale dynamics of sweat droplet evaporation conform with the general heat index criteria reported by the US National Weather Service (NWS), where the background color gradient in Figure 4D indicates the transition between safe (yellow) and dangerous (red) environmental conditions.16 Our findings indicate that elevated humidity levels lead to imperfect evaporation and reduced evaporative cooling and heat transfer, yielding an elevated heat index that translates into critical conditions for the human body’s comfort and well-being.

Our observations show imperfect sweat evaporation under specific temperatures and humidities, explaining the microscale mechanisms behind the feeling temperature or heat index. The findings of this study shed light on how the complex composition of sweat droplets as a biofluid leads to complex evaporation behaviors. We found that the hindered evaporation in sweat droplets can be rooted in perpetuated moisture sorption by sweat deposits on the skin surface. The extended sweat droplet lifetime in a high-humidity environment, despite the temperature conditions, leads to concerns about the ability of the body to thermoregulate via evaporative cooling, which decreases significantly with increasing humidity in different ranges of temperatures. The proposed approach allows experimentally bridging the gap between the physiological events at the macroscale addressed in the previous reports and the microscale dynamics surrounding the imperfect evaporation of sweat droplets and the consequent heat transfer. Such knowledge has significant implications for understanding the disruption of body thermoregulation and assisting in regulating temperature efficiently through advancements in public health care, materials science, and technology. Continuing this research, we highlight the necessity of investigating the impact of environmental wind flow on evaporation dynamics and exploring the influence of different porous substrates on the effects of advanced clothing.

Materials and Methods

Materials

Artificial sweat (Pickering laboratories, 1700–0020 catalog number, USA), deionized water (Deionized water system, ELGA LabWater, UK) droplets, and artificial skin (Strat-M Membrane, Millipore, USA) were used. The artificial sweat used in this work is composed of different amino acids, minerals, metabolites, and other components, with concentrations closely matching experimentally determined values for adult human eccrine sweat. Microscope coverslips (Deckglaser, 18 × 18 mm2, Marienfeld, Germany) were used as substrates for inverted optical microscopy. The glass substrates were ultrasonically cleaned in ethanol (99.5% purity, Sigma-Aldrich, USA) for 10 min and carefully dried with a nitrogen blow gun (99.9% purity) before the sweat or water droplets were placed.

Methods

All droplets were adjusted to 5 μL and gently applied to the surface of the substrates with a micropipette. All of the experiments were conducted in a specially designed chamber with several sensors that allowed us to control the environmental conditions precisely, encompassing humidity and temperature. This chamber also shielded the experiments from potential disturbances such as wind or other unwanted airflow (details are given in Supporting Information, Figure S12). The sensitivity of the sensors allows us to measure temperature and humidity with an error of ±2 °C and ±3%, respectively. The optical microscopy images of the droplets were taken with an inverted optical microscope (CKX53, Olympus, Japan). The side-view microscopy images, droplet radii, contact angles, and surface tensions were measured by using a drop shape analyzer system (DSA25, Kruss, Germany). The viscosities were measured with a viscometer (SV-10, AND, USA). An electronic mass balance (EX224G, Ohaus) with a readability of 0.1 mg was used to record the time-dependent mass changes during evaporation. X-ray computed microtomography was performed at Pohang Light Source II (PLS-II) using the 6C Bio Medical Imaging (BMI) beamline to image the artificial skin structure and the final sweat droplet deposition on its surface. The acquired data was visualized using the commercial software Amira (FEI, Hillsboro, Oregon). Infrared (IR) thermal imaging was performed using a thermal imaging camera (Testo 885, Germany) with an accuracy of 2 °C.

Acknowledgments

This research has been supported by the Amorepacific Corporation.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.est.4c00850.

  • Side view images captured by a drop shape analyzer, initial contact angles of sweat and water droplets on glass and artificial skin substrates, X-ray tomography of artificial skin, phase diagram of final deposit patterns, depinning and receding dynamics of sweat droplets, sweat droplet evaporation dynamics for varying temperatures and humidity, long-lasting imperfect evaporation time-lapse, multiple sweat droplet system dynamics, IR images of sweat droplets at different humidity and temperature, and experimental setup (PDF)

  • Side and top view of evaporating droplets at different environmental conditions, moisture sorption by the sweat deposit due to increasing humidity, and multiple sweat droplets dynamics (ZIP)

Author Contributions

M.B. and M.G. contributed equally to this work. M.B., M.G., and B.M.W. designed research; M.B. and M.G. performed research; M.B. and M.G. analyzed data; and M.B., M.G., and B.M.W. wrote the paper. All authors reviewed the manuscript.

The authors declare no competing financial interest.

Supplementary Material

es4c00850_si_001.pdf (2.3MB, pdf)
es4c00850_si_002.zip (51.3MB, zip)

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