The Physics of Moisture Transport: Capillarity and MMT

A fabric that moves sweat away and dries quickly is not performing magic, it is obeying hydrostatics and surface physics. Liquid moisture transport in a fibrous structure is two distinct events: first wetting, the tendency of liquid to spread over the fibre surface, then wicking, the spontaneous capillary draw of liquid into the gaps between fibres. Polyester itself is hydrophobic; performance comes less from fibre chemistry than from cross-section geometry, yarn architecture and finish. This article treats those mechanisms in the language of standard test methods, with engineering magnitudes.

Wetting: contact angle and the Young equation

When a water droplet meets a surface, the contact angle (theta) it forms is the measure of wetting. By the Young equation that angle arises from the balance of the solid-vapour, solid-liquid and liquid-vapour interfacial tensions. If theta < 90 degrees the surface is hydrophilic and the liquid spreads; if theta > 90 degrees it is hydrophobic and the droplet beads. On bare polyester the water contact angle is typically in the 70-90 degree band, which explains why, without intervention, the fabric tends to repel sweat. For wicking to be possible cos(theta) must be positive, i.e. the contact angle must be below 90 degrees.

Capillary rise: the Washburn equation

The gaps between fibres behave like fine capillary tubes. In a capillary, the penetration distance L is given by the Lucas-Washburn equation: L = sqrt[(gamma * r * t * cos(theta)) / (2 * eta)]. Here gamma is the liquid's surface tension, r the effective capillary/pore radius, theta the contact angle, eta the dynamic viscosity and t time. It is derived from the Young-Laplace capillary pressure (Pc = 2*gamma*cos(theta)/r) and Hagen-Poiseuille flow, and applies to the horizontal case where gravity is neglected.

The equation tells designers three things. First: penetration distance grows with the square root of time (L proportional to sqrt-t), so wicking is very fast at the start and decelerates steadily. Second: a small pore radius r means higher capillary pressure (Pc proportional to 1/r) but slower flow (L proportional to sqrt-r); this trade-off explains why microfilament fineness raises both pull and the need for a balance point. Third: because cos(theta) is a multiplier, any hydrophilic finish that drops the contact angle below 90 degrees directly affects wicking. In vertical wicking gravity opposes the rise, so liquid does not climb forever; there is an equilibrium height where capillary pressure equals the hydrostatic head.

Measuring vertical wicking: strip tests

Capillary rise can be measured directly. In the classic strip test a vertically suspended fabric strip is dipped at its lower end into a water reservoir, and after fixed times the height the water has climbed (mm) is read. DIN 53924 is a short-duration (minutes-scale) height-of-rise method for relatively fast-wicking fabrics; BS 3424 (Method 21) is a resistance test extending up to 24 hours for very slow-wicking coated/technical fabrics. The modern counterpart is AATCC 197: capillary rise on a vertically aligned specimen is tracked against time, which is the geometry closest to the real-world scenario of fabric hanging against sweating skin. For lateral spread, AATCC 198 is used (the time for liquid dropped at the centre to reach a 100 mm diameter); results from the two methods are not comparable because the geometries differ.

MMT: mapping moisture management in one shot

A strip test tells you how fast a fabric transports, but not the directional difference between the two faces of a single fabric. AATCC 195 (Moisture Management Tester, MMT) measures exactly that: the specimen is placed between concentric sensor rings on its top and bottom faces, a test solution (usually a synthetic-sweat saline) is dropped at the centre, and the electrical resistance/conductance of both faces is logged over time. The resulting curve resolves into six indices: wetting time (WT, s, separate top/bottom), absorption rate (AR, %/s), maximum wetted radius (MWR, mm), spreading speed (SS, mm/s), accumulative one-way transport capability R, and the index that combines them, the overall moisture management capability OMMC (a 0-1 index).

OMMC is the single-number summary of a fabric's ability to manage moisture, computed from three components: bottom-surface absorption rate (ARB), one-way transport capability (R) and bottom-surface maximum spreading speed (SSB). The logic is clear: a good sport fabric pushes liquid from the skin side (top) to the outer side (bottom), spreads it over a wide area outside and primes it for rapid evaporation. Each index is converted from value to a 1-5 grade (1 = poor, 5 = excellent). The one-way index R is the difference between the time integrals of water content of the top and bottom faces; a large positive R means a net flow of liquid away from the skin.

Push-pull: dual-layer directional transport

The MMT one-way index R also describes a design goal: unidirectional movement of liquid away from the skin. The construction that delivers this most cleanly is the push-pull dual layer. The skin-facing inner face is deliberately hydrophobic (e.g. polypropylene, or untreated microfilament polyester) while the outer face is chosen hydrophilic. The resulting wetting gradient acts as a one-way pump for the liquid: the hydrophobic inner layer does not hold sweat and, through its small pores, pushes it to the outer layer; the high-absorbency outer layer pulls the liquid out and spreads it over a wide area, enlarging the evaporation surface. This makes the engineering target the surface-energy difference between two layers rather than fibre chemistry; the skin stays dry, evaporation happens outside.

Drying rate: closing the wet zone

Transport is only half the job; comfort is completed when the liquid evaporates away. Drying-rate methods measure different physical quantities and must not be conflated. AATCC 201 (heated plate) dries the specimen on a plate held at 37 degrees C, mimicking skin temperature, with airflow across the top, and derives the drying rate (typically mm/hour or g/m2/hour) from the surface-temperature change; it is the most application-realistic scenario. AATCC 199 instead tracks a fully saturated specimen at 37 degrees in a gravimetric moisture analyser, deriving drying time from weight loss. AATCC 200 estimates dry-out time from surface temperature without external heating, with air drawn through. In short, 201 and 200 are temperature-based and 199 is weight-based; whenever drying is claimed, the method used must be stated.

Hydrophilic finish vs durable hydrophilicity

There are two routes to making the cos(theta) multiplier positive. The first is a chemical finish: applying a hydrophilic polymer (e.g. polyether/PEG derivatives) to the polyester surface lowers the contact angle and quickly improves both absorption rate and wicking. But simple finishes wash out; performance typically drops markedly after 30-50 wash cycles. More durable solutions use multi-point-anchored polyethers with one end covalently bonded to the polyester and the other end permanently hydrophilic, silane coupling agents, or cross-linked polymer networks; well formulated, much of the wicking can survive 30 washes. The second route is structural: a channelled cross-section (e.g. the industry-familiar Coolmax-type grooved filament) or microfilament fineness tunes the capillary r and surface area through geometry; this effect does not change with washing because it depends on shape, not chemistry. The most robust performance usually combines structural geometry with a durable finish.

A designer's checklist

• Measure contact angle first: if it does not drop below 90 degrees the fabric repels sweat, and a hydrophilic intervention is mandatory (Young equation).
• Remember the sqrt-t deceleration of wicking: the first seconds matter most; real comfort hides in the early absorption rate.
• If a single number is wanted on the MMT, ask for OMMC; but R (one-way) is the true proof of a push-pull design.
• Do not compare vertical (AATCC 197) and horizontal (AATCC 198) wicking results; the geometries differ.
• When claiming drying, state the method: AATCC 201/200 (temperature) and AATCC 199 (weight) are not the same number.
• Do not say 'manages moisture' without addressing wash durability: a finish washes out in 30-50 cycles, a structural cross-section does not.