A Curly Q: Is Frizz a Matter of Friction?

Despite a popular interest in how environmental factors influence perception of hair, surprisingly little research exists on the topic—much less any attempting to explain how interface properties such as tactile friction relate to how hair feels by touch. Tactile friction refers to the friction generated upon interaction between a surface and the fingertips and, in other applications, has been shown to influence tactile perception, for example, Skedung et al. (2020). However, to our knowledge, to date no attempts have been made to find relationships between tactile friction and perception on hair.

In this exploratory study, the tactile friction coefficients and perception of bleached (single treatment) and unbleached swatches of straight (Type I) and curly (Type IV) natural hair (Loussouarn et al., 2007) were assessed in a climate-controlled room at all permutations of 80% or 20% relative humidity and 30°C or 15°C ambient temperature (four test conditions). The study was conducted in accordance with the Declaration of Helsinki (2013).

Twenty female participants (mean age = 26.8 years, standard deviation = 3.8 years; n = 5 per condition) placed their nondominant hand inside a box which obscured the hair samples from view. Participants touched each hair sample sequentially in a random order, while rating their frizziness, slipperiness, stiffness, smoothness, volume, electrostaticity, clumping, pleasantness, fly-away, shape recovery, and strand alignment from 0 (Not [descriptor]) to 100 ([descriptor]) on a visual analogue scale using a tablet.

Tactile friction (the friction between an operator’s finger and the hair samples) was measured using a ForceBoard™ (Industrial Dynamics AB, Sweden), a universal friction and force tester that continuously records the frictional force and applied load during interaction. Two measurements in both the root-to-tip and tip-to-root directions (directional anisotropy due to the asymmetric morphology of hair with overlapping scales and cuticle edges was expected—this same construction is responsible for the irritating tendency of wool to shrink on heating) were performed for each sample in all four climate conditions by two operators (see Figure 1). Dynamic friction coefficients were calculated as the ratio of the frictional force and applied load in each data point (1,000 datapoints/s), and the average was calculated. Across all samples and conditions, analysis of variance showed that the average tactile friction was higher in the tip to-root direction than the root-to-tip direction (p < .001).

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Figure 1.

Method used to measure tactile friction, showing a top-down perspective of the ForceBoard™ with a hair swatch clamped in place, an illustration of a friction trace shown on the PC screen, and the four hair samples included. Fifteen strokes were made per measurement. Data in the red areas were used for analysis.Note: Please refer to the online version of the article to view the figure in colour.

Mixed analyses of variance (within: hair shape and treatment, between: temperature and humidity) were used to analyze the perceptual data. Interestingly, ratings of stickiness were higher in lower temperatures (p = .012). It has been suggested that a neural representation of wetness might rely on perceiving coldness and stickiness (Filingeri et al., 2014), and so perceived coldness here may have been somewhat conflated with stickiness. Straight hair was rated slipperier, smoother, and more pleasant than curly hair, while curly hair was rated as stiffer, more voluminous, clumpier, and having more fly-aways (all p < .05).

Since hair is often anecdotally considered to be frizzier in higher humidity, this attribute was of particular interest. Somewhat surprisingly, no effects of humidity, temperature, or treatment on ratings of frizziness were found, although curly hair overall was rated as frizzier than straight (p < .001). However, relative to straight hair, the tip-to-root tactile friction for curly hair increased to a greater extent in higher humidity (p < .001). Analyzing the data separately for curly and straight hair indicated that frizziness ratings of curly hair correlated positively with root-to-tip and tip-to-root tactile friction, which was not found for straight hair (see Figure 2). It may be that elevated humidity disproportionately affected perceived frizziness for curly hair, which seems in good keeping with anecdotal reports.

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Figure 2.

Mean frizziness ratings of straight and curly hair by touch against tip-to-root (top) and root-to-tip (bottom) tactile friction, with 95% confidence intervals shaded. Points represent average values for each combination of temperature, humidity, and treatment (8 points per hair shape).Note: Please refer to the online version of the article to view the figure in colour.

Increased temperature and humidity generally cause hair strands to swell and the cuticle edges to lift, increasing the roughness of the hair surface, the friction coefficient, and its anisotropy. Curly hair tends to have more lopsided cuticles (Bernard, 2003) which are associated with increased directional anisotropy in friction behavior at higher humidity (Mizuno et al., 2013). Therefore, curly hair might be more susceptible than straight hair to perceivable changes in texture due to humidity-induced swelling, as the curls in the hair structure form due to already asymmetrical mechanical stresses. Increased friction of the hair (especially for the curly hair in the tip-to-root direction) may hinder the ability of the hair strands to return to an ordered state after disruption and the curly morphology may exacerbate the effect of increased resistance to reordering as a consequence of a higher friction coefficient.

The results indicate that although higher humidity, often portrayed as immediately causing wild and frizzy hair, affected the tactile friction coefficients for both straight and curly hair, it did not appear to affect tactile perception of frizziness directly or consistently. Instead, relationships between tactile friction and perceived frizziness were detected only in the case of curly hair. It is possible that the greater mechanical disorganization associated with both the humidity and resulting friction may be partly responsible for differences in perceived tactile frizziness for curly hair. However, these are no more than broad brush strokes! Despite the small sample size and exploratory nature of this study, it represents a beginning in the investigation of environment on the tactile perception of hair.