How the Fingerprint Effect Applies to Digitized Fingerprint-Like Structures

The fingerprint effect describes the relationship between slip speed, fingerprint ridge spacing, and the frequency of vibrations created by the movement of a fingerprint across a surface. We have previously shown that the spacing between straight, parallel, evenly spaced ridges in fingerprint-like structures, and thus the vibrations produced by the fingerprint effect, are dependent on the orientation of the ridges with respect to the direction of movement. We also showed that, when ridge orientation is known, the fingerprint effect can be used to estimate slip speed in real-time. The physical processes behind the fingerprint effect also apply to the interaction between a surface and other, non-ridge, microstructures. It is, therefore, theoretically possible to use the fingerprint effect generated by these structures to estimate slip speed. However, it is first necessary to understand the nature of the fingerprint effect generated by these non-ridge structures. In this paper, we show that digitized structures, evenly spaced in columns and rows, have a more complex relationship to the fingerprint effect than ridges do. At most orientations, these structures produce vibrations amplified around four frequencies, each determined by a set of virtual ridges defined by the digitized structures. A sensor with
$\mathbf{100}\ \boldsymbol{\mu} \mathbf{m}$
tall,
$\mathbf{150}\ \boldsymbol{\mu} \mathbf{m}$
wide micropillars in evenly spaced rows and columns, with a spacing of
$\mathbf{300}\ \boldsymbol{\mu} \mathbf{m}$
center-to-center, is fabricated. This sensor is tested at angles between 0° and 90° by 15° increments. The results support our theoretical analysis.