lubricants Article Surface Hierarchy: Macroscopic and Microscopic Design Elements for Improved Sliding on Ice Karlis Agris Gross 1, *, Janis Lungevics 2 , Ernests Jansons 2 , Ilze Jerane 1 , Michael J. Wood 3 and Anne-Marie Kietzig 3   Citation: Gross, K.A.; Lungevics, J.; Jansons, E.; Jerane, I.; Wood, M.J.; Kietzig, A.-M. Surface Hierarchy: Macroscopic and Microscopic Design Elements for Improved Sliding on Ice. Lubricants 2021, 9, 103. https:// doi.org/10.3390/lubricants9100103 Received: 18 June 2021 Accepted: 22 September 2021 Published: 12 October 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Faculty of Materials Science and Applied Chemistry, Institute of Materials and Surface Engineering, Riga Technical University, LV-1048Riga, Latvia; ilze.jerane@rtu.lv 2 Department of Mechanical Engineering and Mechatronics, Faculty of Mechanical Enngineering, Transport and Aeronautics, Riga Technical University, LV-1048 Riga, Latvia; janis.lungevics@rtu.l (J.L.); ernests.jansons_1@rtu.lv (E.J.) 3 Department of Chemical Engineering, McGill University, Montreal, QC H3A 0C5, Canada; michael.wood3@mail.mcgill.ca (M.J.W.); anne.kietzig@mcgill.ca (A.-M.K.) * Correspondence: kgross@rtu.lv Abstract: Frictional interaction with a surface will depend on the features and topography within the contact zone. Describing this interaction is particularly complex when considering ice friction, which needs to look at both the macroscopic and microscopic levels. Since Leonardo da Vinci shared his findings that roughness increases friction, emphasis has been placed on measuring surface coarseness, neglecting the contact area. Here, a profilometer was used to measure the contact area at different slicing depths and identify contact points. Metal blocks were polished to a curved surface to reduce the contact area; further reduced by milling 400 μm grooves or laser-micromachining grooves with widths of 50 μm, 100 μm, and 150 μm. Sliding speed was measured on an inclined ice track. Asperities from pileup reduced sliding speed, but a smaller contact area from grooves and a curved sliding surface increased sliding speed. An analysis of sliding speed versus contact area from incremental slicing depths showed that a larger asperity contact surface pointed to faster sliding, but an increase in the polished surface area reduced sliding. As such, analysis of the surface at different length scales has revealed different design elements—asperities, grooves, curved zones—to alter the sliding speed on ice. Keywords: ice friction; topography; texture; contact area 1. Introduction Ice friction draws on the initial finding by Leonardo di Vinci, that friction between two surfaces varies linearly with the force applied to a surface, acting through the contact area. Recent explanations of ice friction have been interpreted through lower friction from a lubricant film [1] or by considering the thermodynamics at the sliding interface [2]. Contact area from the application of a load has only been approximated, emphasizing the need for renewed efforts to quantify the actual surface contact. The goal of this work was to evaluate the contact area at the macro-scale and the micro-scale to see the effect of sliding over ice and then propose further characterization at the nanoscale. Few ice-friction studies have investigated the effect of the contact area. Initial experi- ments from 1939—in a cave dug out from ice at Jungfraujoch in the Swiss Alps—showed a slight increase in the friction coefficient with contact area, where the contact area varied from 2 to 300 mm 2 [3]. Experiments on a larger tribometer with a more pliable polyethy- lene slider (contact area: 200–1000 mm 2 ) on ice displayed a larger change in friction [4]. Recent experiments on contact area have shown that the increase in contact area of an ice-hemisphere loaded onto a quartz base correlates well with the friction force [5]. On a larger scale—such as with the skeleton—the contact area reduced by bending the runner Lubricants 2021, 9, 103. https://doi.org/10.3390/lubricants9100103 https://www.mdpi.com/journal/lubricants