Advancing and receding angles – Dynamic contact angle measurements on mixed alkyl monolayers

Abstract

Mixtures of 1-decene and methyl-10-undecenoate with different mole fractions were used to prepared mixed monolayers on silicon (1 0 0) surfaces. The formed layers were characterized by dynamic contact angle measurements of two different liquids using the dynamic sessile drop method. The advancing and receding contact angles as well the calculated molar wetting free energies decreased with increasing mole fraction of methyl-10-undecenoate. Otherwise the molar dewetting free energies as well as the contact angle hysteresis stay nearly constant. A difference in the wetting and dewetting properties on mixed monolayers is observed, whereby mainly dispersive interactions are responsible for wetting. Furthermore we show that the contact angle hysteresis is independent of surface roughness, surface composition and surface free energy, which indicates that neither surface roughness nor heterogeneities are responsible for this hysteresis. We attribute the hysteresis to interactions between the liquid and the silicon substrate.

Introduction

Contact angle measurements are of increasing interest in investigations of surface properties like self cleaning surfaces and superhydrophobicity [1], [2], [3], [4], [5]. Here two different variants are commonly used as they give a fast experimental overview of the macroscopic characteristics: static contact angle measurements [6] as well as advancing and retracting contact angle measurements (in the following dynamic contact angle) [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. The preferred method for characterization, the dynamic contact angle measurements, is carried out on bare or monolayer-terminated surfaces. Most investigations were done on monolayers consisting of one type of molecule [7], [8], [9], [10], [11], [12], [13], [14], [15], [16] while measurements on mixed monolayers are quite rare [17], [18], [19], [20], [21], [22], [23], [24]. Extrand [7], [8], [9] performed investigations on the molar free wetting and dewetting energies on monolayer terminated surfaces consisting one type of molecule. Mixed monolayers on gold have been investigated by dynamic contact angle measurements [25] and the results were investigated with respect to the molar free energy [7].

Generally during dynamic contact angle measurements two specific contact angles occur: the advancing and receding contact angle. The advancing contact angle θ_a can be measured by inflating a liquid drop on a substrate surface, while the receding contact angle θ_r arises by shrinking a liquid drop on the surface. The difference between the two angles is known as the contact angle hysteresis.

Different interactions between the liquid and the surface cause contact angle hysteresis. Generally the receding contact angle is smaller than the static contact angle. Compared to the static contact angle the advancing contact angle is either larger or in the cases of polar liquids about the same [20]. In the case of mixed monolayers the fraction of dispersive surface groups can play a role for the advancing contact angle of polar liquids, since these groups prevent an unhindered spreading of the polar liquid. On the other hand polar groups support a spreading of the polar liquids. Consequently, polar groups determine the receding contact angle, since the polar liquid sticks on these groups [26]. In contrast for dispersive liquids the polar groups are crucial for the advancing contact angle and the dispersive groups for the receding contact angle. Beside this also roughness and heterogeneity of a surface can influence the contact angle and as it has been shown that also perfectly flat and homogenous surfaces are characterized by contact angle hysteresis [9], further parameters has to be taken into account.

In the present work a series of mixed 1-decene (DEC)/methyl-10-undecenoate (MUD) monolayers on silicon were prepared and characterized by dynamic contact angle measurements. These films are prepared by covalent attachment of alkenes to a bare silicon surface, which is different from often described monolayers on gold or oxide surfaces [9], [19], [27], [28]. The alkyl monolayer formation is well-known and can be realized e.g. by thermal reaction [19], [27], [28], [29]. By using terminal functionalized alkenes (e.g. carboxyl, hydroxyl, amino or ester groups) the properties of the monolayer can be easily modified which enables a wide variation of the surface properties [29], [30], [31]. Additionally the surface properties can be tuned gradually by mixing different alkenes [32]. Such mixed monolayers can act as nanostructures in hierarchical-structured surfaces which are known to influence the properties of liquids and the underlying microstructured substrate [33].

The aim of this study is to investigate the dependence of the dynamic contact angles and the associated molar wetting energies on the distribution of MUD within the DEC layer. Therefore we studied a series of monolayers with a different mixing ratio between DEC and MUD with the dynamic sessile drop method.