Adhesion Hysteresis

Definition

Adhesion hysteresis describes the phenomenon where taking apart two contact surfaces dissipates more energy than bringing the surfaces together.

Scientific Fundamentals

The adhesion phenomenon exists widely in atomic force microscope (AFM)-based nanotribology studies (Qian and Xiao 2000). As shown in Fig. 1, when the AFM tip contacts to the silicon surface (loading) and then pulls off (unloading) from the sample, the adhesive force F _a can be obtained from the force-distance curve. The surface energy has a positive effect on F _a.

Adhesion Hysteresis, Fig. 1

Force-distance curve of approaching and separating process

Theoretically, the work of adhesion W or surface tension (surface energy) γ, where W = 2γ, is normally viewed as the reversible work done on bringing two surfaces together or the work needed to separate two surfaces from contact. But under most realistic conditions, it will dissipate more energy to take apart two contact surfaces than bring two surfaces together, which can be defined as adhesion hysteresis (Bhushan 2001). Further understanding of the molecular mechanisms underlying this phenomenon is essential for explaining many adhesion phenomena, energy dissipation during loading-unloading cycles, contact angle hysteresis, and the molecular mechanisms associated with many frictional processes.

Adhesion hysteresis may be thought of as being due to mechanical or chemical effects, namely the physical and chemical changes process is irreversible during the contact-separating process. In general, if the energy change, or work done, on separating two surfaces from adhesive contact is not fully recoverable on bringing the two surfaces back into contact again, the adhesion hysteresis may be expressed as (Chen et al. 1991)

$$ \begin{array}{llll} \qquad \qquad{W_{\text{R}}}{ }\quad>\quad { }{W_{\text{A}}} \cr \qquad {\rm{ receding\quad\quad advancing}} \cr {\rm{ (separating) \quad\quad(approaching)}} \cr {\rm{ or \qquad}}\Delta W = { }{W_{\rm{R}}} - {W_{\rm{A}}}{ } > 0\end{array} $$

(1)

where W _R and W _A are the adhesion or surface energies for receding (separating) and advancing (approaching) solid surfaces, respectively.

Adhesive hysteresis can also be found in measurement of contact angles. For example, when the liquid spreads and then retracts from a surface, the advancing contact angle θ _A is generally larger than the receding angle θ _R (Bistac et al. 1998). The contact angle θ is related to the liquid-vapor surface tension γ _L and the solid-liquid adhesion energy W by the following equation:

$$ {\text{(1 + cos}}\theta {)}{\gamma_{\text{L}}}\ { = }\ W $$

(2)

It can be concluded that wetting hysteresis or contact angle hysteresis (θ _A > θ _R) implies that either γ _L,A > γ _L,R or W _R > W _A. The wetting hysteresis or contact angle hysteresis actually implies the adhesion hysteresis as (1) presents.

Molecular dynamics (MD) simulations (Landman et al. 1990) have predicted hysteresis in the force versus tip-sample distance related to intrinsic mechanical instabilities at the atomic scale. As the tip approaches the sample, the interaction is essentially given by the attractive conservative forces (i.e., the hysteretic force of adhesion is equal to zero). Just before contact, there is a sudden jump of the interaction force due to the formation of an atomic scale connective neck and, as the tip retracts, there is an additional adhesive force that drops approximately linearly in a few interatomic distances (D ₀). This behavior associated with the formation and rupture of a solid neck (Landman et al. 1990) is similar (except for some oscillations due to atomic rearrangements) to the capillary-induced liquid bridges (Sahagún et al. 2007). So a linear adhesive force (when the tip retracts) can be simply estimated (Köber et al. 2008) for D _ts < D ₀ by

$$ {F_{\text{hys}}} \approx \frac{{2\Delta E}}{{D_0^2}}({D_{{ts}}} - {D_0}) $$

(3)

where F _hys is the hysteretic force of adhesion and ΔE is the energy dissipated in the contact process.

Energy dissipating processes originate from practical constraints of the finite time of measurements and the finite elasticity of materials, which prevent many loading-unloading or approach-separation cycles from being thermodynamically reversible (Bhushan 2001).

Key Applications

Silicon-based micro/nanoelectromechanical systems (MEMS/NEMS) experience oxidation during micromachining and subsequent exposure to air, forming a surface of increased stiction and friction (Chandross et al. 2004). In these applications, the surface forces, such as friction and adhesion, play a much more important role than the bulk forces because of the surface and size effects in nanoscale. For instance, the adhesion has induced the negative influence on the efficiency and reliability of the digital micro-mirror devices (Bhushan and Liu 2004). Therefore, with the development in MEMS/NEMS, the understanding and control of the adhesion and friction performance have become an important issue of concern. Further study on adhesion hysteresis can help in understanding the correlation between adhesion and friction performance in real application of MEMS/NEMS, which can shed new light on the design of demanded operating surface of devices.

Figure 2 shows that the adhesion energy hysteresis (Δγ) has a direct impact on the friction between two interactional surfaces in movement, namely two surfaces with high adhesion hysteresis will lead to a high friction during movement. On the other hand, surface with low surface energy or adhesion will produce less adhesion hysteresis and thus low friction will be obtained in a nanoscale test. This is of significance for contact surfaces of MENS/NEMS, which can easily suffer from friction and adhesion in operations.

Adhesion Hysteresis, Fig. 2

Contact radius vs. applied load (r³–L) curves measured from two fluid-like monolayer-coated surfaces in inert air (a) and decane vapor (b). The solid lines are obtained from the analysis of these data using the JKR equation where R = R ₁ R ₂/(R ₁ + R ₂) (R ₁ and R ₂ stand for the radii of two spheres), K is bulk elastic moduli, and γ is surface energy. For dry monolayers (a) the adhesion energy on unloading (γ_R = 40 mJ/m²) is greater than that on loading (γ_A = 28 mJ/m²), and the adhesion energy hysteresis can be calculated as Δγ = γ _R–γ _A = 12 mJ/m². For monolayers exposed to saturated decane vapor (b), the adhesion hysteresis disappeared (From (Chen et al. 1991). With permission)

In practice, a modified surface with low adhesion usually exhibits low friction and good antiwear performance. As shown in Fig. 3 (Yu et al. 2009), dual-layer films of STA/APS, PPA/APS, and PAA/APS were grafted to the Si(111) surface. The adhesion force between the AFM tip and sample surface was measured and recorded in Fig. 4. It was found that for all samples, the adhesion force tested in air was higher than that in vacuum, especially for the hydrophilic Si–OH surface. The hydrophilic Si–OH surface showed the largest variation in its adhesion force, which decreased from 74.8 nN in air to 30.2 nN in vacuum. This is because, in air, water meniscus may form on samples depending on their hydrophilicity and humidity, which may further induce an additional capillary adhesion between AFM tip and sample (Qian et al. 2003).

Adhesion Hysteresis, Fig. 3

Schematic structures of self-assembled films on Si(111) substrate (Figs. 3–5: From Yu et al. 2009. With permission)

Adhesion Hysteresis, Fig. 4

Adhesion forces of Si with native surface, Si–OH surface, and self-assembled films measured in air and in vacuum (Yu et al. 2009)

Adhesion Hysteresis, Fig. 5

Friction force versus the applied load curves of Si, Si–OH surface, and self-assembled films measured in (a) air and (b) vacuum (Yu et al. 2009)

Considering that the adhesion of samples was smaller when measured in vacuum than in air, it can be inferred that these films will exhibit accordingly lower friction in a vacuum test based on preceding discussions of adhesion hysteresis. Figure 5 illustrates the friction force versus normal force curves of Si, Si–OH, and self-assembled films measured in air and in vacuum. The dual-layer film STA/APS constructed by densely packed long chains revealed much lower friction force than PPA/APS film by poor-packed short chains in both air and vacuum (Fig. 5a, b) (Xiao et al. 1996). Compared with the PPA/APS film with the tail methyl groups, the PAA/APS film presented a relatively high friction force resulting from the distortion and rotation of phenyl groups.

In summary, a comprehensive understanding the adhesion hysteresis can help us predict the magnitude of friction force. Minimizing adhesion hysteresis can effectively improve the micro- and nanotribological performance of contact surfaces during operation, which is practical in designing new contact surfaces, such as self-assembled film-coated surfaces, of MENS/NEMS devices.

Cross-References

Basic Concepts in Adhesion Science

Interfacial Energy

Liquid Contact Angle Measurement

Self-Assembled Monolayers

Surface Forces, Surface Tension, and Adhesion

Surface Free Energy