By the Numbers: The Tug of Water – Why Some Layers Hold Sweat and Others Let it Go

Introduction

During hard exertion, the body can produce sweat faster than it can evaporate. Liquid sweat begins to collect on the skin, and clothing must move that water away from the body through one or more layers.

A few months ago, I evaluated three base-layer garments and found clear performance differences. But one key question remained unanswered:

Which fabrics are most effective at moving liquid sweat away from the skin and into the next layer?

To answer that question, I developed a test specifically designed to track liquid water as it moves through a simplified clothing system. The test measures how much water remains on the “skin,” how much is retained in the base layer, and how much reaches the layer above.

In other words, it answers a simple but previously unmeasured question:

When skin is wet, where does the water actually go?

Figure 1 shows the test device used in this study.

As testing progressed, I expanded the scope from three fabrics to eight, representing a broader range of base-layer designs. With that expanded set, consistent patterns began to emerge.

The testing reveals several key patterns:

• Hydrophobic fabrics require sufficient pressure to initiate liquid transfer through their pore structure
• Pore structure strongly influences how effectively hydrophobic fabrics move liquid once transfer begins
• Hydrophilic fabrics rapidly absorb liquid from the skin but do not necessarily release it easily. Transfer depends on sufficient driving forces   –  primarily the pressure between layers and the receiving layer’s ability to draw in water. As the receiving layer becomes wetter, that ability decreases, reducing transfer. This behavior contrasts with the common expectation that wicking fabrics continuously move moisture away from the skin.

Together, these behaviors help explain a common experience: two garments may both be marketed as “moisture managing,” yet behave very differently once sweat becomes liquid. Some continue to move water away from the skin, while others become saturated and uncomfortable.

This article focuses on understanding why that happens. By tracking liquid water as it moves  – or fails to move  – between layers, the testing provides a clearer picture of how base-layer fabrics behave under high-exertion conditions.

How to Read This Article:

This article does three things:

1. Introduces a new test method,
2. Explains the physics of hydrophobic vs hydrophilic behavior, and
3. Presents experimental results.

That is a lot, but you will learn a lot about how base layers function.

If you want a quick takeaway read:

• How Hydrophobic and Hydrophilic Fabrics Transfer Liquid Moisture
• Case 1 Results
• Conclusions

For a deeper understanding, read the full text. The appendices provide additional detail. Alternatively, read a section or two at a time and then come back to it. There is a lot here, but I think these tests raise the bar on our insights into how base layers work.

Author’s Note:
The results and conclusions presented here apply to the eight fabrics tested in this study. I do not claim that these findings extend to all hydrophobic or hydrophilic base-layer fabrics. However, the observed behaviors are consistent with well-established physical principles described in the scientific literature, beginning with Young and Laplace (1805) and later formalized by Washburn (1921). These foundational studies describe the capillary physics that underpin modern understanding of liquid transport in fibrous materials.

How Hydrophobic and Hydrophilic Fabrics Transfer Liquid Moisture

Hydrophobic fabrics can get wet, and they can transfer water effectively

Hydrophobic base layers are often described as “staying dry,” and in one narrow sense, that’s true   –  the fibers themselves do not attract or absorb water. But during hard exertion, many hikers have experienced something that seems to contradict this idea: hydrophobic layers can feel heavy, clammy, and very wet.

The reason is simple but often overlooked. Fabrics are mostly empty space. Even when the fibers repel water, liquid sweat can still collect inside those empty spaces. In other words, a hydrophobic garment can hold a surprising amount of liquid water without the fibers ever becoming “wet.”

Figure 2 shows this clearly. After high-intensity activity in cold conditions, large amounts of liquid water can be seen within two hydrophobic garments. In the infrared images, darker blue regions indicate areas of high water content. Despite being made from hydrophobic fibers, both garments contain substantial amounts of trapped liquid sweat. The Brynje mesh shirt (right) retained 87 g of sweat (65% of its dry weight), while the Alpha Direct shirt trapped 62 g (55% of dry weight).

Whether a hydrophobic garment stays relatively dry or becomes saturated depends on a simple balance: how fast liquid sweat enters the fabric versus how quickly it can move through and out. If liquid water passes through the fabric as fast as it arrives, little accumulates. If transfer is slower than incoming sweat, liquid gradually fills the pore spaces, and the garment becomes increasingly wet  – even though the fibers remain hydrophobic. In that condition, the fabric may feel clammy, heavy, and provide reduced insulation.

In my testing, hydrophobic fabrics do not absorb water through capillary action. That distinction is important. Capillary-driven uptake occurs when fibers attract water molecules to their hydrogen bonding sites. True hydrophobic fibers lack these bonding sites and therefore cannot generally pull water into the fabric on their own.

Instead, hydrophobic fabrics require external pressure to force liquid water into their pore structure. If available pressures are too low, a hydrophobic fabric can act like a water barrier. As pressure increases, that same fabric may suddenly begin to pass water through.

There is a minimum pressure required to force water into the pores of a hydrophobic fabric. The classic work of Laplace and Washburn, cited above, describes how this pressure can be calculated for a single capillary. I’ll refer to this threshold as the breakthrough pressure.

In real use, this pressure can come from many sources: compression from outer layers or pack straps, movement of the body against clothing, tension in tight-fitting garments, friction between layers, or simply the weight of accumulating liquid sweat. Sliding, stretching, and rubbing can generate brief, localized pressures that momentarily force water into pore openings. Until that pressure threshold is reached, water may remain pooled on the skin.

What determines how much pressure is required? According to Laplace and Washburn (cited above), breakthrough pressure depends largely on two factors: the size of the pore opening and the contact angle between water and the fiber surface. There are other factors that influence breakthrough pressure in a fabric, but, as we shall see, our calculations based on pore-opening size and estimated contact angle correlate well with actual fabric performance.

My testing confirmed that the largest pore openings corresponded to reduced breakthrough pressure. Smaller openings require more pressure to force water through them. At the same time, my testing showed that the breakthrough pressure increased with a higher contact angle.

What is Contact Angle?

Contact angle is a measure of a fiber’s resistance to water. It is measured by how a water droplet sits on a fabric surface. When a water drop deposited on a fabric surface spreads out to form a flattened drop, the surface is hydrophilic and supports capillary wicking. When a water drop beads up into a tall dome, the surface is hydrophobic and resists wetting. Contact angles below about 90° show hydrophilic behavior, while angles above 90° indicate hydrophobic or water-repellent behavior. Many hydrophobic base layers fall in the range of roughly 95° to 115°, whereas strongly hydrophilic materials may exhibit very low angles, approaching zero.

Pore Size and Pathways

It is important to note that apparent pore size alone (measured under the microscope) does not determine how easily liquid passes through. The actual pathways inside a textile are three-dimensional and often constricted where yarns cross or partially block pathways. As a result, fabrics that look very open can still resist breakthrough, while others with smaller visible openings may allow flow if their internal pathways are better connected.

This framework helps explain why hydrophobic base layers behaved so differently in the tests  – and why they may behave differently in real use. One fabric may transfer sweat efficiently and feel relatively dry during intense activity, while another may allow sweat to accumulate on the skin or within the fabric, leading to discomfort  – even though both are made from water-repellent fibers.

Key Takeaways:

1) Hydrophobic fabrics can become wet due to water stored in open pores.

2) In this test, hydrophobic fabrics do not transfer sweat until sufficient pressure forces water into their pore structure.

3) Both pore structure and fiber chemistry greatly influence the pressure required to initiate flow.

4) If water enters a hydrophobic base layer’s pores faster than it leaves, the fabric will retain water and become wet.

Hydrophilic Fabrics: Familiar Strengths, Important Limits

Hydrophilic base layers behave as most hikers already recognize. When liquid sweat appears, these fabrics absorb it immediately, pulling moisture off the skin and spreading it through the fiber and yarn structure by capillary action. This behavior – often described as “wicking” – is discussed in detail in earlier articles, Why Is My Wicking Layer Wet? and How Do Moisture-Wicking Fabrics Work? , which I won’t repeat here.

What matters for this article is a less intuitive point: the same capillary forces that make hydrophilic fabrics absorb sweat so readily also make them inclined to hold on to it.

In a hydrophilic fabric, liquid water is drawn into the pores and distributed through the fabric structure by capillary forces. Once absorbed, that water is not free to move unless another force acts on it. Transferring liquid water to the next layer, therefore, results in a competition between the base layer’s tendency to retain water and the receiving layer’s ability to draw it away.

When the receiving layer is dry, it can readily draw water from the base layer. But as the receiving layer accumulates moisture, its ability to pull additional water decreases. As a result, liquid transfer can slow dramatically  – or stop altogether  – even while the base layer remains wet.

I observed this behavior across all experiments conducted for this article. In two tests, the receiving layer water absorption capacity was reduced in different ways. In each case, less water was transferred from the hydrophilic base layer to the receiving layer. In these tests, the performance of hydrophobic fabrics was largely unchanged and exceeded that of the hydrophilic samples. In two other tests, the hydrophilic base layers simply transferred less water than the hydrophobic samples (with two hydrophobic exceptions). These results are explored in detail in the results section.