Friction and Adhesion Hysteresis between Surfactant Monolayers in Water

Briscoe et al. investigated boundary layers of amphiphilic surfactants in air and water using SFB.

“Briscoe et al., using an SFB, investigated the behaviour of boundary layers consisting of such amphiphilic surfactants both in air and when immersed in water [42, 91].”

When water is added, friction between surfactant-bearing surfaces falls by at least two orders of magnitude, and each surfactant layer becomes thicker by ~0.25 nm.

“When water is added, however, two striking effects are noted. Firstly, the friction between the surfaces falls by at least two orders of magnitude, as seen in trace C of Fig. 13. Secondly, the interference fringes shift, indicating that each surfactant layer has become thicker by some 0.25 nm.”

In air, two monolayer-bearing surfaces come into adhesive van der Waals contact, and friction is larger than applied shear forces, so surfaces do not slide but move in tandem.

“In air the two monolayer-bearing surfaces come into adhesive van der Waals contact over a contact area A (insets to Fig. 13), and the friction F s between them is larger than can be overcome by the shear forces applied as the top surface is moved laterally. Because of this the surfaces do not slide past each other, but rather move back and forth in tandem, as seen in trace B of”

Briscoe et al. showed surfaces separate/rejoin at mid-plane but slide at substrate interface; they adhere at one plane but slide at another.

“Briscoe et al. [42, 91] were also able to show that when the surfaces are separated (and then rejoined) they come apart and together again at the mid-plane, but when sliding they slide at the substrate interface. In other words, the two surfaces adhere at one plane, but slide at another.”

The surfactant used was a double-tailed cationic surfactant with structure [CH3(CH2)10]2N+(CH3)2Br-, whose positively charged ammonium group attaches to negatively charged mica, exposing alkyl tails.

“The top trace A in Fig. 13 shows, as in earlier figures, the applied back-and-forth lateral motion applied to the top surface of the SFB, with the surfaces each bearing a monolayer of a double-tailed cationic surfactant of structure [CH3(CH2)10]2N + (CH3) 2Br -. The positively charged ammonium group attaches to the negatively charged mica surface, while the alkyl tails are”