Supplementary information

Figure 1 presents the results of friction force measurements between two different polymer-coated surfaces.

In this section, it will be explained in detail how these experiments were carried out. Hereafter the main

differences between the friction measurements presented in this correspondence and the friction

measurements presented in the letter by Deng et al.1 are briefly discussed.

Method

Friction force measurements between a polymer-coated silicon substrate and a polymer-coated micrometer

sized silica particle were performed in a fused silica liquid cell using an atomic force microscope (AFM),

Multimode Nanoscope III PicoForce (Bruker, USA). Colloidal probe cantilevers were prepared by attaching a

d=7 µm silica particle (G. Kisker GmbH, Germany) to the end of an uncoated tipless cantilever (CSC12 F-lever,

MikroMasch, Estonia) with the aid of an Eppendorf 5171 micromanipulator, a Nikon Optiphot 100s reflection

microscope and a small amount of thermosetting resin (Shell Epikote 1009). The exact lateral dimensions of the

cantilevers and the sizes of the attached silica particles were determined employing image analysis Vision

Assist 8.0 (National Instruments). Before particle attachment, the normal and torsional spring constants of the

cantilevers were determined by the methods described by Sader et al.2 and Green et al.,3 respectively. The

normal photodetector sensitivity was obtained from the constant compliance region in a force curve between

uncoated surfaces4 and the torsional photodetector sensitivity was determine by the method of tilting the

head as suggested by Pettersson et al.5

Friction studies were performed by running the AFM in “contact mode” using a scan direction perpendicular to

the orientation of the cantilever. In order to achieve information of the friction versus load, lateral force

(friction) data were recorded as a function of first increasing and subsequently decreasing applied load, over a

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scan size of 2 µm with a shear velocity of 2 µm/s. At each applied load, 16 scan lines (each containing 512

points) in each direction (trace and retrace) were recorded (see example in figure S1) and the data from these

scan lines were used to calculate the average friction force at each given value of applied load by:

����� � ��2

���

1����

where Ffric is the friction force, ΔV is the difference in lateral photodetector signal between trace and retrace

(see figure S1), kt is the torsional spring constant, δ is the torsional photodetector sensitivity and heff is the

effective probe height (particle diameter plus half the thickness of the cantilever). As explained in relation to

figure S1, the average friction force obtained at each load is based on 13181 data points (after removing the

points around the reverse in sliding direction) and the standard deviations are given as error bars in figure 1.

These error bars are thus a measure of how much the average friction force varies from the local friction

responses at each surface position.

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Figure S1: The figure displays an example of how the lateral photodetector signal, which is related to the torsional cantilever deflection, is changing when the probe is first moved in one direction along the surface (trace) and subsequently moved in the opposite direction (retrace). As the probe slides over the surface, the torsional deflection (the friction force) is almost constant with only minor local variations. However, as the sliding direction is reversed, the photodetector signal change sign and after a short sliding distance a new steady state is reached. To avoid any influence from the change in sliding direction, 50 data points before and after the change in sliding direction was omitted in the calculation of the average friction force. This cut is marked by the two vertical stipulated lines. The data shown in this figure is obtained at an applied load of 8 nN in the study of friction between surfaces coated by adhesive diblock copolymers (example 2).

Example 1: friction between surfaces coated with methylcellulose

Material:

Methylcellulose, which is a modified cellulose, was obtained as a freeze-dried sample from Akzo Nobel,

Stenungsund, Sweden. The polymer was purified from eventual by-products by use a procedure described

elsewhere, and the resulting polymer was found to have an average molecular weight of 530 kDa.6-8

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A piece of an oxidized silicon wafer and a silica colloidal probe mounted on an AFM-cantilever were

hydrophobized by exposing the surfaces to vapour of (3,3-dimethylbutyl)dimethylchlorosilane (DDS) in a

desiccator overnight. The hydrophobized surfaces were then rinsed repeatedly with water and ethanol.

Friction measurement:

The surfaces for friction measurement were obtained by injecting a 40 ppm solution of methylcellulose in

water into the AFM liquid cell containing the hydrophobized substrate and the hydrophobized colloidal probe

cantilever. Next, the system was left for adsorption for 45 min whereafter the cell was flushed with Milli-Q

water to remove any non-adsorbed and loosely bound material. Then, the friction measurements were

performed as described in the method section. In a previous study, a similar surface preparation inside a QCM-

D cell provided, from Voigt modeling, a layer thickness of 13 nm.9

Example 2: friction between surfaces coated temperature responsive diblock copoolymers

Material:

The diblock copolymer, poly(N-isopropylacrylamide)48-blockpoly((3-acrylamidopropyl)trimethylammonium

chloride)20, was synthesized by atom transfer radical polymerization as described in detail elsewhere.10 The

polymer consists of a cationic block which adsorbs strongly to negatively charged silica, and a temperature

responsive block commonly known as PNIPAAM which obtains a hydrated coiled conformation at low

temperature and a collapsed conformation at high temperature. For the present diblock copolymer adsorbed

to a silica surface, this transition has been shown to start at around 30 ⁰C.11

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Friction measurement:

The surfaces for friction measurements were obtained by injecting 50 ppm of the diblock copolymer in

aqueous solution containing 0.1 mM NaCl into the AFM liquid cell containing a pure silica substrate and a pure

silica colloidal probe cantilever. Then, the solution was left at a temperature of 25 ⁰C for adsorption for 40 min

and the cell was subsequently flushed with 0.1 mM NaCl solution in order to remove any non-adsorbed and

loosely bound material. Previous adsorption studies have demonstrated that the adsorbed amount at this ionic

condition is approximately 0.6 mg/m2, giving a coverage corresponding to moderately stretch but not fully

brush-like conformation of the PNIPAAM chains. The effective layer thickness was from a combination of

results from QCM-D and reflectometry estimated to 3.7 nm.11 After adsorption and rinsing with NaCl solution,

the temperature was raised to 45 ⁰C by use of a thermal application controller attached to a Bioheater element

(Bruker, USA) mounted under the sample. At this elevated temperature where the PNIPAAM chains have

achieved a collapsed conformation, the friction measurements were performed as described in the method

section.

Friction between molecular layers versus between a solid tip and a crystalline layered material

In the study by Deng et al.1 it was nicely demonstrated that the friction between an AFM tip and layered

graphite can increase upon decreasing load when the tip-graphite adhesion is stronger than the inter-layer

adhesion in graphite. In this correspondence, I have by two examples shown that such a situation not is

isolated to well-defined crystalline layered material but is a more general phenomenon related to hysteresis in

the adhesive interaction between two sliding bodies. It should however also be noted that there exist some

fundamental differences between the two examples presented here and the system studied by Deng et al. One

of the main differences is that an adsorbed molecular layer can exhibit lateral mobility as a response to both

the applied normal load and the shear stress. This means that the distribution of molecules around the probe-

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sample contact zone might change during a loading-unloading cycle, which can then change the contact

mechanics and contribute to the hysteresis in the adhesive interaction. This is indeed the case in example 1

where the sudden increase in friction upon loading, seen in figure 1, is a result of an irreversible reorganization

of the molecular layer. For example 2, such reorganization is on the other hand not apparent and the friction

measured in a loading-unloading cycle is almost fully reversible. This means that repeated loading-unloading

cycles provide similar results as the loading-unloading cycle presented in figure 1. However, if the unloading

was reversed to loading before complete surface detachment the friction was again decreased with a rate

similar to (but not identical to) upon unloading. Compared to the situation of a well-defined crystalline layered

material, the situation with adsorbed molecular layer also makes it more difficult to quantify the relative

difference in the adhesion between the two interaction layers and the layer and the substrate/probe,

respectively. The layer does not necessarily detach as a unity but can be lifted from the substrate with bridging

polymer chains spanning the space between the layer and the substrate. Such a situation can lead to a

continuous breaking and reformation of polymer-substrate bonds and thus a new high energy dissipating

mechanism which can contribute to the increase in friction force during unloading. However, if such a

mechanism is responsible or partly responsible for the hysteresis of the friction in the loading-unloading cycles

presented here is unknown.

References

1. Deng, Z., Smolyanitsky, A., Qunyang, L., Feng, X.-Q. & Cannara, R. J. Nature Mater. 11, 1032-1037 (2012).

2. Sader, J.E., Chon, J.W.M., Mulvaney, P. Rev. Sci. Instrum. 70, 3967-3969 (1999). 3. Green, C.P., Lioe, H., Cleveland, J. P., Proksch, R., Mulvaney, P., Sader, J.E. Rev. Sci. Instrum. 75, 1988-

1996 (2004). 4. Thormann, E., Pettersson, T., Claesson, P. M. Rev. Sci. Instrum. 80, 093701 (2009). 5. Pettersson, T., Nordgren, N., Rutland, M.W. Rev. Sci. Instrum. 78, 093702 (2007). 6. Bodvik, R., Karlson L., Edwards, K., Eriksson, J., Thormann, E., Claesson, P.M. Langmuir 28, 13562-13569

(2012). 7. Bodvik, R., Macakova, L., Karlson, L., Thormann, E., Claesson, P. Langmuir 28, 9515-25 (2012). 8. Bodvik, R., Thormann, E., Karlson, L., Claesson, P.M. RSC Advances 1, 305-14 (2011). 9. Bodvik, R., Thormann, E., Karlson, L., Claesson, P.M. Phys. Chem. Chem. Phys. 13, 4260-4268 (2011). 10. Kjøniksen, A.-L., Zhu, K., Pamies, R., Nyström, B. Phys. Chem. B 112, 3294-9 (2008). 11. Dedinaite, A., Thormann, E., Olanya, G., Claesson, P.M., Nyström, B., Kjøniksen, A.-L., & Zhu, K. Soft

Matter 6, 2489–2498 (2010). 6 NATURE MATERIALS | www.nature.com/naturematerials

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