Grip as Needed, Glide on Demand: Ultrasonic Lubrication for Robotic Locomotion

Mostafa A. Atalla; Daan van Bemmel; Jack Cummings; Paul Breedveld; Michaël Wiertlewski; Aimée Sakes

cs.ROFeb 17, 2026

Grip as Needed, Glide on Demand: Ultrasonic Lubrication for Robotic Locomotion

Authors: Mostafa A. Atalla, Daan van Bemmel, Jack Cummings, Paul Breedveld, Michaël Wiertlewski, Aimée Sakes

Organizations: Department of BioMechanical Engineering, Delft University of Technology (TU Delft), 2628 CD Delft, The Netherlands. · Department of Cognitive Robotics, Delft University of Technology (TU Delft), 2628 CD Delft, The Netherlands.

Abstract

Friction is the essential mediator of terrestrial locomotion, yet in robotic systems it is almost always treated as a passive property fixed by surface materials and conditions. Here, we introduce ultrasonic lubrication as a method to actively control friction in robotic locomotion. By exciting resonant structures at ultrasonic frequencies, contact interfaces can dynamically switch between "grip" and "slip" states, enabling locomotion. We developed two friction control modules, a cylindrical design for lumen-like environments and a flat-plate design for external surfaces, and integrated them into bio-inspired systems modeled after inchworm and wasp ovipositor locomotion. Both systems achieved bidirectional locomotion with nearly perfect locomotion efficiencies that exceeded 90%. Friction characterization experiments further demonstrated substantial friction reduction across various surfaces, including rigid, soft, granular, and biological tissue interfaces, under dry and wet conditions, and on surfaces with different levels of roughness, confirming the broad applicability of ultrasonic lubrication to locomotion tasks. These findings establish ultrasonic lubrication as a viable active friction control mechanism for robotic locomotion, with the potential to reduce design complexity and improve efficiency of robotic locomotion systems.

Figures & tables

Fig. 1: Concept of ultrasonic lubrication for locomotion. (a) Conventional nature-inspired friction control methods for locomotion through normal force modulation, contact sequencing, or anisotropic surface features. (b) The proposed approach uses transverse ultrasonic vibrations to actively control friction at the contact interface. These vibrations pressurize the trapped fluid between the microscopic roughness asperity junctions, creating a pressurized fluid cushion that disengages the two surfaces, allowing for controllable switching between high-friction ‘grip’ (vibrations off) and low-friction ‘glide’ (vibrations on) states to drive locomotion.

Fig. 2: Design and integration of cylindrical and flat friction control modules for bio-inspired locomotion. (a) Cylindrical module: a $10~\mathrm{mm}$ ring-shaped resonator with four bonded piezoelectric plates, supported at nodal lines by an internal frame to minimize interference with oscillation. Finite element simulation predicted the second flexural resonance mode at $22.7~\mathrm{kHz}$ . (b) Flat module: a $32\times 10\times 1~\mathrm{mm}$ slider resonator with piezoelectric plates bonded at antinodes and supported via a ball-joint groove at nodal lines. Simulation results predicted the third flexural resonance mode at $21.2~\mathrm{kHz}$ . (c) Inchworm-inspired locomotion system: two cylindrical modules connected by a push–pull cable-sheath mechanism and actuated by a double-acting cylinder, enabling bidirectional motion through selective activation of each module. (d) Ovipositor-inspired locomotion system: two flat modules, one fixed and one mounted to a linear actuator, enabling bidirectional motion through selective activation of the movable module during the motion cycle.

Fig. 3: Experimental setup for the friction characterization experiments. A miniature load cell mounted along the sliding axis measured friction forces via a bracket guided on a horizontal stage, while the slider was mounted to a vertical linear guide for motorized horizontal motion and free vertical motion. A 100 g weight ( $\approx$ 1 N) provided the normal load. Two protocols were tested: (i) sliding with and without continuous vibration at a voltage amplitude of 140 V to assess stability of the lubrication effect when activated, and (ii) sliding with the excitation voltage amplitude ramped up from 0 to 140 V to evaluate dynamic friction modulation. Tests were performed on dry and wet PLA, sandpapers of different grit sizes, granular soil, and ex-vivo porcine intestinal tissue.

Fig. 4: Vibration characterization of cylindrical and flat friction control modules. (a) Frequency response showing resonance peaks at $22.9~\mathrm{kHz}$ for the cylindrical module and $21.4~\mathrm{kHz}$ for the flat module. (b) Vibration amplitude as a function of applied voltage, exhibiting linear growth up to an amplitude of $\approx 4~\mu\mathrm{m}$ at 100 V. Both modules achieved vibration amplitudes exceeding $2~\mu\mathrm{m}$ at ultrasonic frequencies ( $\geq 20~\mathrm{kHz}$ ), fulfilling the design requirements of ultrasonic lubrication.

Fig. 5: Time-lapse sequence of the inchworm locomotion system traversing a rigid PLA track. By selectively activating ultrasonic lubrication at each friction control module, forward propulsion is achieved following the inchworm motion cycle. Images are shown at one-second intervals. Refer to supplementary videos 1 & 2 for demonstrations of the use cases presented in this paper, including the inchworm locomotion case.

Fig. 6: Friction modulation experiments across a range of surfaces. (a) Measured friction forces under forward and backward sliding show that the vibrating slider effectively reduces friction on PLA (dry and wet), soil, sandpaper, and biological tissue (colon). (b) Voltage modulation of the coefficient of friction (top row) and corresponding friction reduction percentage (bottom row). On PLA, reduction is consistent with air squeeze-film formation and exhibits nonlinear saturation. On wet PLA, ultrasonic lubrication enabled friction reduction via liquid squeeze-film formation, following a more linear trend compared to air. Sandpaper surfaces show higher reduction for finer grit (240) compared to coarser grit (150), due to enhanced squeeze-film effects in smaller asperities. Soil produced relatively low friction reduction owing to its unstable granular contact. On colon tissue, viscoelasticity damped vibration energy, decreasing friction reduction. (c) Summary of friction reduction percentage on different substrates during experiments on forward–backward sliding at maximum voltage (left) and experiments with voltage modulation (right).

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Characterization and Correlation of Robotic Snake Scale Friction and Locomotion Speed

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Department of Mechanical and Industrial Engineering, University of Massachusetts, Amherst, MA, USA.

Robot Locomotion Soft Robotics

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Snake robots are inspired by the ability of biological snakes to move over rock, grass, leaves, soil, up trees, along pavement and more. Their ability to move in multiple distinct environments is due to their legless locomotion strategy, which combines distinct gaits with a skin that exhibits frictional anisotropy. Designing soft robotic snakes with similar capabilities requires an understanding of how this underlying frictional anisotropy should be created in engineered systems, and how variances in the frictional anisotropy ratio affect locomotion speed and direction on different surfaces. While forward and backward frictional ratios have been characterized for previous scale designs, lateral friction and the associated ratios are often overlooked. In this paper, our contributions include: (i) the development of a novel articulated pseudo-skin design that is modular, easy to construct and has removable or replaceable scales; (ii) experimental measurement of the frictional characteristics of otherwise-identical scales at varying angles of attack (15°, 25°, 35°, 45°) on different surfaces of interest (grass, bark, smooth surface, carpet);(iii) separate measurements of locomotion speed for each angle and surface. Consequently, while we observed some consistent trends between frictional coefficients and scale angle, aligning with literature and intuition, we were not able to consistently identify expected correlations between frictional ratios and locomotion speed. We conclude that either frictional ratios alone are not sufficient to predict the observed speed of a snake robot, or that specific measurement approaches are required to accurately capture these ratios.

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