Biotribology 26 (2021) 100170

Available online 17 February 20212352-5738/© 2021 Elsevier Ltd. All rights reserved.

Amphiphilic gel lubrication and the solvophilic transition

Eric O. McGhee a, Allison L. Chau b, Megan C. Cavanaugh c, Jose Gabriel Rosa d, Cullen L. G. Davidson IV e, Jiho Kim f, Juan Manuel Uruena f, Brent S. Sumerlin e, Angela A. Pitenis b, W. Gregory Sawyer a,f,g,*

a Dept. of Materials Science and Engineering, Herbert Wertheim College of Engineering, University of Florida, Gainesville, FL 32611, United States of America b Materials Department, College of Engineering, University of California, Santa Barbara, Santa Barbara, CA 93106, United States of America c Department of Chemical Engineering, College of Engineering, University of California, Santa Barbara, Santa Barbara, CA 93106, United States of America d Department of Chemical Engineering, Herbert Wertheim College of Engineering, University of Florida, Gainesville, FL 32611, United States of America e Department of Chemistry, College of Liberal Arts and Sciences, University of Florida, Gainesville, FL 32611, United States of America f Dept. of Mechanical and Aerospace Engineering, Herbert Wertheim College of Engineering, University of Florida, Gainesville, FL 32611, United States of America g J. Crayton Pruitt Family Dept. of Biomedical Engineering, Herbert Wertheim College of Engineering, University of Florida, Gainesville, FL 32611, United States of America

A R T I C L E I N F O

Keywords: Friction pHEMA Ethanol

A B S T R A C T

Lubrication in biology uses lipids, proteins, and aqueous gels to maintain hydration and provide low shear stress over a range of sliding speeds and contact pressures. The unquestionably amphiphilic nature of proteins and the complexity found in the aqueous solutions suggest that these systems operate near an optimal solvophilic con-dition. To explore the potential for a solvophilic transition in an amphiphilic gel, we perform tribological and swelling measurements of poly(hydroxyethyl)methacrylate, pHEMA, equilibrated over a range of water-ethanol solutions. Depending on the ethanol concentration, Gemini pHEMA gels achieve either low friction (μ < 0.02) and low adhesion or high friction (μ > 1) and high adhesion. We hypothesize that as the solution becomes increasingly ethanol-rich the alkyl regions of ethanol more fully associate with the aliphatic regions of pHEMA, effectively coating the chains with a hydroxyl presenting surface, promoting hydrogen-bonding and the influx of water and leading to maximum in swelling and mesh size, leading to a dramatic reduction in friction and adhesion. We suggest that the tribological behaviors of amphiphilic Gemini gels reflect the presentation of hy-drophobic and hydrophilic domains across the interfaces during sliding. These experiments explore the lubri-cation and solvophilic transitions in amphiphilic Gemini gels and suggest fundamental mechanisms and solution composition through which biotribological joints leverage lipid and protein-based complex fluids to achieve lubricity.

1. Introduction

Synovial fluid is a lubricant in load-bearing joints that has complex rheological behaviors that remain the subject of as much speculation today as it was in 1968 when Walker, Dowson, Longfield and Wright published their manuscript “’Boosted Lubrication’ in Synovial Joints by Fluid Entrapment and Enrichment” [1]. The view of synovial fluid lubri-cation has evolved over the past 50 years and models of the lubrication in synovial joints have shifted to a gel-like boundary lubrication model, where a “supramolecular synergy” between hyaluronan, lubricin, and phospholipids act together to maintain hydration and lubrication [2]. The lubricating mechanism in synovial joints is thought to involve

confinement and fluid pressurization in cartilage, where the low vis-cosity of the aqueous phase at high shear-rates are responsible for the low shear stress, friction, and wear rate of the system [3–6]. Finally, development efforts in bioinspired gels and surface-attached brushes have made tremendous advances in biotribology and offer the promise to improve health by providing durable, resilient, aqueous lubrication to biological applications [7–9].

Proteins are the building blocks of life, and cells secrete these mac-romolecules in abundance for cellular signaling, construction of extra- cellular matrices, and lubrication [10,11]. For lubrication, these mac-romolecules are often comprised of amphiphilic protein backbones with carbohydrate groups decorating the length of the macromolecule to

* Corresponding author at: Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL 32611, United States of America. E-mail address: wgsawyer@ufl.edu (W.G. Sawyer).

Contents lists available at ScienceDirect

Biotribology

journal homepage: www.elsevier.com/locate/biotri

https://doi.org/10.1016/j.biotri.2021.100170 Received 19 December 2020; Received in revised form 8 February 2021; Accepted 11 February 2021

Biotribology 26 (2021) 100170

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promote hydration. The mechanisms associated with organizing the macromolecules into gels are thought to rely on weak bonds across end- groups, or hydrophilic interactions across domains along the backbone. The aqueous solutions are predominantly water with polyelectrolytes and also contain soluble hydrophilic molecules such as phospholipids and small proteins thought to assist in organization and lubrication through bonding and passivation of hydrophilic domains respectively. As an example, the lipid-protein interactions in synovial fluid are thought to reduce friction through protein interactions [12–14], and changes to these interactions through shifts in concentrations of lipids and proteins in the synovial fluid are clinically relevant markers in arthritis. A loss of lipids in synovial fluids may have serious impact in chronic tissue damage and inflammation [15]. Combination of hyal-uronic acid with synovial fluid relevant lipids have shown enhanced protective outcomes to tissue damage in rabbits with transected anterior and posterior cruciate ligaments when compared to synovial protein injection alone [16].

Hydrogels are often discussed in cartilage and synovial lubrication and have been used as cartilage mimics in research and implants [17–20] but tuning a solution to the amphiphilic nature of the polymer to optimize lubricity has not been explored. We hypothesize that the solutions in synovial lubrication and tear films are optimized for lu-bricity within an amphiphilic gel network., To demonstrate this effect, we have performed experiments on a well-known amphiphilic gel, pHEMA, in a simple amphiphilic solution comprised of water and ethanol. Although pHEMA is a classic example of a hydrogel, in the absence of incorporated carboxyl groups from methacrylic acid or other hydrophilic functionalities, pHEMA gels typically contain less than 40% water, have low permeability, and are mechanically tough with an elastic modulus on the order of MPa. Friction studies on neat pHEMA gels in water are limited, but our experiences reveal friction coefficients on the order of μ ≈ 0.4. In this manuscript we began with pHEMA gels in a Gemini configuration submerged in water, and then performed ex-periments after equilibrating the gels in solution with increasing con-centrations of ethanol. Remarkably, the finding revealed either low friction (μ ≈ 0.01) or high friction (μ > 1) and high adhesion, depending on the ethanol concentration. The sensitivity of this amphiphilic Gemini gel system to the addition of ethanol to water was profound and had a cross-over behavior (from high to low friction) after a critical concen-tration of ethanol necessary to passivate the hydrophobic domains was exceeded. The lowest friction was found for the most swollen gels, which were equilibrated in solutions containing both water and ethanol. This simple system of neat pHEMA, water, and ethanol (Fig. 1a–b) points to a plausible mechanism from which amphiphilic gels and solutions can synergistically develop low friction configurations by eliminating favorable hydrophobic/hydrophobic interactions in aqueous solutions by solution passivation of these domains – a plausible mechanism of lubrication in biotribogical joints using protein-based rheological fluids.

2. Materials and methods

2.1. Amphiphilic gel preparation

Poly(hydroxyethyl)methacrylate (pHEMA) probes and disks were prepared by polymerizing (hydroxyethyl)methacrylate (HEMA) (67 wt %), N,N′-methylenebisacrylamide (MBAm) (0.3 wt%), ammonium per-sulfate (0.15 wt%), and tetramethylethylenediamine (0.15 wt%) in ul-trapure water under a controlled cover gas of nitrogen. Polyolefin molds with a 2 mm radius of curvature were used to create hemispherically- capped probes, and the 60 mm diameter disks were about 5 mm thick and molded between parallel polystyrene plates. The polyolefin and polystyrene casting surfaces had average surface roughness on the order of Ra ≈ 20 nm. Prior to testing, pairs of pHEMA gel probes and disks were equilibrated for >48 h in individual containers and solutions of ethanol and ultrapure water at specified concentration of ethanol by volume (0%, 10%, 20%, 25%, 32%, 35%, 50%, 75%, and 100% ethanol)

at room temperature.

2.2. Swelling experiments

Twenty-two cylindrical disks of pHEMA gel (10 mm diameter, 5 mm thickness) were prepared following the methods above and equilibrated in sealed vials of ethanol solutions (0 to ≈100 vol% ethanol) maintained at room temperature. After 48 h, the final sample dimensions were compared to initial measurements to calculate the sample volume change (Fig. 2a). As shown in Fig. 1(a), the swelling of the pHEMA disks showed a monotonic increase with increasing ethanol volume fraction up to about 30 vol% ethanol, after which the swelling behavior shifted to an upward concave relationship with a maximum around 50 vol% ethanol (gels in a 50% ethanol solution had a volume increase of about 275%). For solutions above 80 vol% ethanol, the gels had a reduced capacity for swelling; the swelling for the ≈100 vol% and 20 vol% ethanol solutions were comparable. This upward concave behavior of pHEMA swelling in ethanol/water solutions has been observed previ-ously [21–23].

2.3. Calculations of mesh-size and volume fractions

As the gels swell, the polymer volume fraction decreases, the solution volume fraction increases, and the characteristic mesh size, ξ, increases (Fig. 2b,c). The experimental data, models and procedures outlined by Peppas et al. provide an estimate of the characteristic mesh size for these pHEMA gels [24]. The volume changes of the pHEMA gels during swelling in ethanol/water solutions are expressed a swollen fraction, Vs/ Vo, and can be used to model the change in polymer concentration, ν, during swelling as given by Eq. 1, where the subscripts o and s represent the initial and swollen state respectively.

νs = νo/(Vs/Vo) (1)

The mesh size, ξs, for the pHEMA gels can then be estimated following the analysis and models given by Peppas et al. [24], Eq. 2,

Fig. 1. a) Chemical structures of pHEMA, ethanol and water. b) Proposed in-teractions of ethanol and water with pHEMA chains: hydrogen bonding of pendent hydroxyl groups with water and ethanol, hydrogen bonding between water and ethanol, and hydrophobic interactions between ethanol and the pHEMA backbone.

E.O. McGhee et al.

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where (

r20

)1/2

is the end-to-end distance in un-perturbed state, and

based on our crosslink density has a value of (

r20

)1/2

∼ 15 A.

ξs = ν− 1/3s

(

r20

)1/2

(2)

After crosslinking, the end-to-end distance in the un-perturbed state is unchanged, and the mesh size in the swollen state can be found by combining Eq. (1) and Eq. 2, as given in Eq. (3).

ξs = νo− 1/3(Vs/Vo)

1/3(

r20

)1/2

(3)

2.4. Friction and indentation

Tribological experiments of self-mated (“Gemini”) pHEMA gels were performed with a custom-built linear reciprocating biotribometer pre-viously described in Pitenis et al. [25,26]. All experiments were per-formed in the Gemini configuration with an average normal load of Fn ≈

1 mN and a constant sliding speed of v = 100 μm/s. The targeted stroke length of 2 mm was used for all experiments except for the 10–30 vol% ethanol samples due to excessive friction; these samples had a stroke length of 5 mm to ensure sliding. Contact indentation experiments were also performed in the Gemini configuration and on the biotribometer with a maximum normal load of Fn ≈ 1.5 mN and a constant indentation

rate of vind = 1 μm/s. The maximum force of adhesion, Fadh, was calculated using traditional methods as described by McGhee et al. [27] for this instrument. All gel experiments were performed fully submerged in their respective ethanol/water solution.

2.5. Hydrodynamic lubrication

Models of soft-elastohydrodynamic lubrication (soft-EHL) developed by Hamrock and Dowson [28] were also a subject of much discussion with Prof. Dowson regarding the lubrication of gels but have consis-tently predicted friction coefficients below μ = 0.002, shear stresses below 10 Pa, and film thicknesses of the order of 30 nm. The soft-EHL models generally predict friction coefficients an order-of-magnitude below the experimentally-observed values of friction coefficient for Gemini gel contacts at slow sliding speeds, and the suggestion is that operating lubrication for such contacts is not hydrodynamic.

3. Results

The tribological behavior of amphiphilic pHEMA gels in Gemini contact showed a dramatic sensitivity to ethanol concentration (Fig. 3a). In ultrapure water, the friction coefficient of self-mated pHEMA was moderate, about μ ≈ 0.35. However, equilibration in a solution of 10 vol % ethanol provoked a remarkable order-of-magnitude increase in fric-tion coefficient (μ > 4). As the ethanol content in the solution increased, the friction coefficient decreased monotonically, going to μ ≈ 0.02 around 50 vol% ethanol and remained low through ≈100 vol% ethanol.

Fig. 2. Swelling behavior and properties of pHEMA gels in a binary solution of water and ethanol. All figures use the binary solution’s volume percent of ethanol as the x-axis, with the remainder of the solution being ultrapure water: a) Swelling of pHEMA gels after 48 h submerged in volume fractions of ethanol. b) Polymer volume fraction as a result of swelling. c) Calculated mesh size of the pHEMA gel using the methods in [24] based on swelling measurements in (a). d) Dynamic viscosity of water-ethanol solutions; viscosity data from [33].

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One suggestion for the source of shear stress and dissipation is that the fluid is being sheared and penetrating across a single mesh-size on each surface [29] (i.e. τ ~ η V/(2ξ), where η is the viscosity of the so-lution, 2ξ is the combined distance over each surface of hydrodynamic penetration, and V is the sliding speed). Fig. 3b plots the friction coef-ficient versus the ratio of viscosity/mesh size and clearly demonstrate a lack of correlation.

The large range of friction coefficients of μ > 1 suggests a strong adhesive contribution to friction, and a series of experiments were performed on the same instrument prior to friction experiments (Fig. 3c). Interestingly, the force of adhesion was nearly negligible be-tween the self-mated pHEMA interface in ultrapure water, Fadh ≈ 1 μN but jumped nearly three orders of magnitude for the 10 vol% ethanol solution, Fadh > 600 μN. The trend of adhesion force as a function of ethanol concentration in the solution followed the friction coefficient behavior, and the adhesion monotonically decreased with increasing ethanol content up to 50 vol% ethanol. The suggestion from these measurements is that the ethanol additions promote a strong adhesive component during sliding that is absent in pure water and minimized around 50 vol% solution.

4. Discussion

The water-swellability that results from the presence of pendent hydroxyl moieties and the polarity of the methacrylic ester groups re-sults in pHEMA generally being regarded as a hydrophilic polymer. However, given the additional presence of less-polar aliphatic regions, pHEMA is inherently amphiphilic. In fact, even when uncrosslinked, high molecular weight pHEMA is generally only water-swellable, not fully water-soluble, due to intermolecular and intramolecular hydro-phobic associations of the aliphatic regions of the polymer preventing molecular dissolution [30]. Provided sufficient equilibration time, the addition of water-miscible alkyl alcohols to water-swollen pHEMA gels can induce dissociation of these hydrophobic associations and promote an influx of water.

We speculate that these solvent-induced transitions in inter- and intra-gel associations are responsible for the swelling of pHEMA gels, as well as the friction and adhesion behaviors of Gemini pHEMA gels. The surface chemistry and presentation of hydrophobic and hydrophilic domains in pHEMA gels are the result of molecular-scale rearrange-ments that have occurred during long-term (>48 h) equilibration in different solutions of ethanol and water. Given that the time scales for complete segmental rearrangement and functional group presentation are much greater than the short dynamics of friction and adhesion, we reason that the tribological behaviors of Gemini gels are measures of this phenomenon and reflect the presentation of hydrophobic and hydro-philic domains across the interfaces during sliding.

The equilibration of pHEMA gels in water leads to equilibrium sur-faces that are highly hydrophilic due to the presentation of the polar regions and aggregation-induced shielding of the aliphatic regions of pHEMA (Fig. 4a). This view of the surface functionality is consistent with the low adhesion observed in Gemini contacts of pHEMA that have been stored, aged, and tested in pure water separately and then brought into contact for adhesion testing. Similarly, the friction coefficients of the specimens stored and aged separately, and then brought into contact for friction testing while submerged in pure water were consistent with a non-adhesive elastomer. The addition of ethanol in the solution allows partial dissociation and plasticization of the hydrophobic associations, which leads to sufficient mobility for the expression of hydrophobic domains to the gel surface (Fig. 4b). This is consistent with the work of Li et al. [31] who found that solvent interactions and reduction in barriers to rotational and translational motion of the chain segments impacted adhesion in hydrogels. We hypothesize that during friction and adhesion testing, these exposed hydrophobic regions will spontaneously de-wet [32] in contact, and these strong hydrophobic-hydrophobic in-teractions in water are responsible for both the high friction and high

Fig. 3. Frictional behavior of pHEMA gels in binary solution of water and ethanol. a) Friction coefficient vs. percent ethanol in solution. Self-mated pHEMA gels exhibit moderate friction in 0 vol% ethanol (100% water) solu-tions, but the friction coefficient rapidly increases by over an order of magni-tude with the addition of just 12 vol% ethanol to the solution. Subsequent increases of volume fraction of ethanol in solution rapidly decreases friction, even approaching superlubricity with μ ≈ 0.025 at 75 vol% ethanol in solution - associated friction loop shown in inset. b) Friction coefficient vs. viscosity/mesh size is shown with associated vol% ethanol in parenthesis above the data point. c) Maximum force of adhesion vs. volume percent ethanol in solution. Adhesion forces were negligible in water but jumped to nearly 650 μN at 12 vol% ethanol. Subsequent increases in volume fraction of ethanol in solution rapidly decrease adhesive forces, finally reaching a minimum in a 100 vol% ethanol solution. Inset: data presented in semi-log.

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adhesion. The maximum friction coefficient and maximum adhesion force for Gemini contacts of pHEMA were observed in the 10 vol% ethanol solution, which was the lowest concentration of ethanol tested above pure water. For the 10 vol% ethanol solution we hypothesize that there was sufficient ethanol for the partial dissociation and plasticiza-tion of the hydrophobic association to present hydrophobic domains on the surface but insufficient ethanol in solution to passivate these regimes and prevent de-wetting and strong adhesion during contact and sliding.

As the solution becomes increasingly ethanol-rich, we reason that the alkyl regions of ethanol more fully associate with the aliphatic regions of

pHEMA, effectively coating the chains with a hydroxyl presenting sur-face, promoting hydrogen-bonding and the influx of water and leading to maximum in swelling and mesh size (Fig. 4c). The rehydration of these surfaces leads to a dramatic reduction in friction and adhesion.

Given the inherently amphiphilic nature of pHEMA, it is not surprise that maximum swelling is observed in a solvent mixture of water and a polar organic solvent. Extrapolating beyond these specific results, most charge-neutral synthetic polymer gels are similarly composed of polar and aliphatic molecular regions. It is reasonable to expect that similar interfacial and tribological phenomena might be observed in these cases as well. The curious result of moderate friction and low adhesion for the Gemini pHEMA gels in pure water may be explained through kinetics. These gels and their surfaces were formed in an aqueous process and stored separately for days prior to testing. The resulting surfaces are unquestionably hydrophilic as observed through contact angle measurements.

In an effort to explore the role of kinetics directly, we loaded the pHEMA gels in Gemini contacts submerged in water to a contact area of ~0.5 mm2 and held these samples in contact for different amounts of time from ~1 s to ~100,000 s. The adhesion force increases mono-tonically with contact time as shown in Fig. 5a. The conceptual frame-work to describe this mechanism is that the pHEMA gels that are polymerized and aged in water have already organized a hydrophilic exterior surface with hydrophobic interactions preferentially expressed in the interior of the sample. Therefore, for a new hydrophobic/hydro-phobic interaction to occur across the contact, the chains must (1) spontaneously break an interior hydrophobic/hydrophobic interaction, (2) maintain the surface hydrophobic domain expression for a sufficient amount of time that it can (3) match with a chain on the countersurface that has undergone the same process. It is therefore not surprising that this process would be slow and would have a low probability of success. Following a simple model, assuming first order kinetics, we assume that the fractional coverage, θ, of hydrophobic interactions across the interface is zero at time zero, θ = 0 at t = 0, and that once an association is formed it remains intact until the force of adhesion separates the chains during unloading. The fractional coverage of bonds as a function of time is then given by Eqs. (4) and (5).

dθ/dt = k (1 − θ) (4)

θ = 1 − e− k t (5)

Assuming that each chain within a single mesh could be involved in a single hydrophobic association, the number, N, of hydrophobic in-teractions can then be estimated by dividing the projected contact area, Ao, by the projected area of one mesh-size, π ξ2, and multiplying this number by the fractional coverage of bonds, θ, (Eq. 6).

N =(Ao/(

π ξ2) ( 1 − e− k t) ) (6)

The force of adhesion, Fadh, is then assumed to be proportional to the number of hydrophobic associations, N, and the time dependent force can then be given by Eq. 7, where Fo is the adhesion force at time zero and F∞ is the force at infinite time or complete saturation of hydro-phobic interactions at the surface (i.e. θ = 1).

Fadh = Fo +(F∞ − Fo)(1 − e− k t) (7)

Plotting the natural log of 1 minus the normalized force of adhesion,

ln(

1 − F)

, where the force of adhesion normalized by fit values for Fo

and F∞ (F = (Fadh − Fo)/(F∞ − Fo)), versus contact time, Fig. 5b gives an exponential decay constant of k = − 1.3 × 10− 5 s, which suggest that the time constant for this process, τ = 1/k, is on the order of ~80,000 s. Given the lack of solvation in the vicinity of the intra-gel hydrophobic associations present in gels aged in water, this time scale is consistent with the expected slow evolution of gel surfaces that requires hydro-phobic regions of the outermost chains to randomly present themselves

Fig. 4. Schematic depiction of pHEMA chain configuration upon equilibration in various concentrations of ethanol/water solvent mixtures. a) When no ethanol is present (i.e., 100% water), pHEMA selectively presents hydrophilic pendent polar groups to the aqueous solvent while the aliphatic hydrophobic backbones are associated via inter- and intra-chain associations. These hydrated surfaces lead to low friction domain. b) At intermediate concentrations, aliphatic ethanol selectively solvates and plasticizes the hydrophobic domains of pHEMA, but incomplete passivation due to low ethanol content exacerbates chain-chain and Gemini gel interactions, causing a marked increase in friction and adhesion. c) In ethanol-rich solutions, the presence of sufficient amphi-philic solvent passivates the remaining hydrophobic regions leading to reduced interfacial interactions between Gemini gels.

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to a complementary exposed hydrophobic region on the mating gel. Based on the apparent area of contact, Ao ~ 0.5 mm2, and the pro-

jected area of one mesh-size, π ξo2 ~ 10 nm2, the number of hydrophobic

interactions could be on the order N = 5 × 1010. Assuming a saturation force of F∞ = 25 mN, the force for each interaction is on the order of 0.5 pN, which is consistent with the forces for weak bonds and within the range of forces one might expect for a short-ranged hydrophobic interaction.

Moreover, while not investigated here, the entropic contributions to swelling that result from solvation, combined with the important role of hydrogen-bonding suggests that temperature will play an important part in dictating the lubrication and solvophilic transitions in Gemini gels and biotribological interfaces.

5. Concluding remarks

These experiments explore the solvophilic transition using the amphiphilic gel of poly(hydroxyethyl)methacrylate, pHEMA, equili-brated over a range of water-ethanol solutions. Below the solvophilic transition, the Gemini pHEMA gels had high friction and high adhesion; above the solvolphilic transition, the Gemini pHEMA gels had low friction and low adhesion. We hypothesize that below the solvophilic transition, the aliphatic ethanol selectively solvates and plasticizes the hydrophobic domains of pHEMA, but incomplete passivation exacer-bates chain-chain interactions causing a marked increase in friction and adhesion. As the solution becomes increasingly ethanol-rich, the alkyl regions of ethanol more fully associate with the aliphatic regions of pHEMA, effectively coating the chains with a hydroxyl presenting sur-face. This promotes hydrogen-bonding and the influx of water, leading to a maximum in swelling and mesh size and a dramatic reduction in friction and adhesion. Broadly, these findings suggest potential roles for soluble proteins and lipid moieties in biotribology.

Declaration of Competing Interest

The authors have no conflict of interest.

Acknowledgements

We remember our dear friends Duncan and Mabel Dowson and the numerous discussions of biotribology over quiet walks in the

immaculately maintained garden and afternoon tea. Mabel’s amazing amphiphilic shortbread recipe, which she called “the rules of three”, is best enjoyed with a proper cup of Yorkshire Gold Tea: “3 sugar, 6 butter, 9 flour. Mix together to make a dough, shape in a pan, then a slow oven for an hour”.

These experiments were performed for Duncan and were partially supported by Alcon Laboratories and the National Science Foundation (NSF) Materials Research Science and Engineering Center (MRSEC) at UC Santa Barbara through DMR-1720256 (IRG-3). The use of the shared facilities of the MRSEC is gratefully acknowledged; the UCSB MRSEC is a member of the Materials Research Facilities Network (www.mrfn.org). E.O.M. and A.L.C. acknowledge the support of the National Science Foundation Graduate Research Fellowship Program under Grant No. 2017242875 and 1650114, respectively.

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