Feeling small: exploring the tactile perception limits

Abstract

The human finger is exquisitely sensitive in perceiving different materials, but the question remains as to what length scales are capable of being distinguished in active touch. We combine material science with psychophysics to manufacture and haptically explore a series of topographically patterned surfaces of controlled wavelength, but identical chemistry. Strain-induced surface wrinkling and subsequent templating produced 16 surfaces with wrinkle wavelengths ranging from 300 nm to 90 μm and amplitudes between 7 nm and 4.5 μm. Perceived similarities of these surfaces (and two blanks) were pairwise scaled by participants, and interdistances among all stimuli were determined by individual differences scaling (INDSCAL). The tactile space thus generated and its two perceptual dimensions were directly linked to surface physical properties - the finger friction coefficient and the wrinkle wavelength. Finally, the lowest amplitude of the wrinkles so distinguished was approximately 10 nm, demonstrating that human tactile discrimination extends to the nanoscale.

Figure 1. Schematic diagram outlining the fabrication procedure of wrinkled surfaces.

(1) Original PDMS substrate with initial length L. (2) Stretching of the PDMS by ΔL using a strain stage. (3) Surface treatment of the PDMS either by UVO or OP treatment. (4) Spontaneous wrinkling occurrence after releasing the pre-strain. (5) Replica moulding of the wrinkled PDMS substrate onto an UV-curable adhesive.

Figure 2. Wrinkled-patterned surfaces ranging from nanometers to micrometers.

Representative 3D images of sinusoidal-patterned surfaces of identical material spanning two orders of magnitude in wavelength (λ) with varying amplitudes (z in this figure) fabricated by the combination of surface oxidation, surface wrinkling, and replica molding techniques (Fig. 1): (a) WS2, (b) WS8, (c) WS11, and (d) WS15 (for details of surfaces included, see Table 1S). (e) An approximate colour scale for the representation of wavelengths.

Figure 3. 2D INDSCAL solution and interpretation of dimensions.

(a) Two-dimensional tactile space (for the group of the first 10 participants) based on perceived similarities among 18 surfaces; the closer the points in the map, the more similar the surfaces are perceived. (b) Finger friction coefficient versus wrinkle wavelength. Colour symbols are based on wrinkle wavelength (red is smallest and blue largest wavelengths; open symbols are “blank” reference surfaces), for details see Table 1S. The point distributions in (a) and (b) are distinctly similar, suggesting that friction and wrinkle wavelength are cues for surface similarity (a third order polynomial fits these data well). The WS1 (λ = 270 nm) surface was not perceived as different to the reference surfaces (BS1 and BS2), whereas the WS2 (λ = 760 nm) and WS3 (λ = 870 nm) surfaces were. The respective amplitudes of the latter two are 13 nm and 22 nm, respectively. The data in (b) are presented as the arithmetic mean ± s.d.

Figure 4. Perceptual quantities (2D INDSCAL) compared to physical quantities.

(a) Dimension 1 vs. finger-friction coefficient and (b) Dimension 2 vs. wrinkle wavelength. Sigmoid functions fit the data well: (a) r² = 0.96 and (b) r² = 0.95, encircled stimuli, exclusive. The two sigmoid functions imply high sensitivity in the middle of the two perceptual ranges. The two asymptotes would indicate sensor saturation in each of the psychophysical plots (a) and (b).