Mechanism of Meniscus Oscillations and Stripe Pattern Formation in Langmuir−Blodgett Films

Abstract

The mechanism of the meniscus oscillations and the stripes formation within the deposited fatty acid monolayer is theoretically analyzed on the basis of a supposition of concentration polarization within the solution during the deposition process. The concentration polarization can lead to decrease of adhesion work, dynamic contact angle, and maximum deposition speed under dynamic conditions resulting in meniscus instability. The adhesion work is evaluated from the disjoining pressure isotherm at a given subphase composition taking into account the charge regulation for a fatty acid monolayer. The relation of the proposed mechanism to the known experimental facts and observations is discussed.

Ion redistribution and meniscus stability at Langmuir monolayer deposition

“…Thus, the supposition of the effect of concentration polarization on critical velocity of deposition correlates with the experimental data about the dependence on the solution composition. Estimations of the magnitude of the maximum deposition rate according to the concentration polarization mechanism give the limiting velocity of the order, which also correlates with the experimental data [28,30].…”

“…The case of fatty acid monolayers in the presence of bivalent counterions, which can bind to the surface, was considered recently in Ref. [28] using the same approach, as proposed by Payens, and the following expression has been derived…”

“…The deposition process is similar also to a diffusion-limited electrochemical reaction where the counterions bdischargeQ (adsorb) at the oppositely charged belectrodeQ (monolayer). Accordingly, the formation of ion concentration profiles near three-phase contact line can be expected during monolayer deposition [28][29][30], typically for membrane and electrode systems.…”

“…Recently Mahnke et al [31] observed meniscus oscillations (so-called bslip-stickQ behaviour) during deposition of arachidic acid monolayers which have been explained by the meniscus instability due to concentration polarization near the contact line [28]. During the withdrawal the threephase contact line follows the substrate sticking on it at a certain position and slips down periodically to other sticking position.…”

“…Our recent investigations of the Langmuir wetting process have demonstrated that the concentration polarization (local change of concentrations and potential at dynamic conditions) develops within the liquid film near the three-phase contact line, especially in the case of small contact angles [27][28][29][30]. At some specific experimental conditions (pH and bivalent ions concentration in solution) this effect can lead to meniscus instability and to a stripelike composition of the deposited fatty acid monolayer [31].…”