3.2 How to Construct Nanomaterials with Superwetting Surfaces
3.2.1 Theoretical Basis of Wettability of Solid Materials
the wetting behaviour of a liquid on a solid surface is a function of the interfacial free energies among the solid/liquid (γSL), solid/vapour (γSV) and liquid/vapour (γLV) interfaces. this can be described by the liquid contact angle (Ca) and the classic model by thomas Young when a liquid droplet is presented onto an ideal flat solid surface (Figure 3.1a and eqn (3.1)).28 that is to say, the wettability of an ideal flat solid surface is determined by the surface chemical composition. Commonly, when the solid surface shows a liquid Ca less than 90°, this solid material is defined as lyophilic, when the solid surface shows a liquid Ca more than 90°, this solid material is defined as lyophobic. For lyophilic materials, cos θCa-ideal > 0, γSV > γSL, these materials commonly have a high surface energy. For lyophobic mate- rials, cos θCa-ideal < 0, γSV < γSL, these materials commonly have a low surface energy.
cos θCa-ideal = (γSV − γSL)/γLV (3.1) however, most of the real solid surfaces are not truly flat, but relatively rough. therefore, the wettability of a solid surface is determined by both the surface chemical composition and the surface roughness factor. For a solid material with a rough surface, the classic Wenzel model and Cassie model are proposed to explain and evaluate the wettability. in the Wen- zel model, the liquid completely pins the grooves of the rough surface in
a wet-contact state where they contact as illustrated in Figure 3.1b and described by eqn (3.2).29
cos θCa = r cos θCa-ideal = r(γSV − γSL)/γLV (3.2) where θCa is the apparent liquid Ca in the Wenzel mode, θCa-ideal is the liquid Ca for an ideal flat solid surface, and r is the surface roughness factor. as r is always larger than 1, the surface roughness factor will provide an enhanced surface wettability and even generate a superlyophobic state with the liq- uid Ca more than 150° or a superlyophilic state with the liquid Ca less than 5°. in brief, for a lyophilic solid material, θCa < θCa-ideal. For a lyophobic solid material, θCa > θCa-ideal.
another classic wetting model is the Cassie model, as illustrated in Figure 3.1c. Vapour is assumed to be trapped in the grooves of the rough surface underneath the liquid, giving a composite surface. in this composite state, the vapour parts of the surface can be considered perfectly non-wetting, the liquid is thus assumed to contact the surface through the top of the asperi- ties in a non-wet-contact mode. in the Cassie model, the wettability of a solid material can be correlated to the chemical heterogeneity of the rough surface and described by eqn (3.3).30
cos θCa = fS(rS cos θCa-ideal + 1) − 1 = fS[rS(γSV − γSL)/γLV + 1] − 1 (3.3) Figure 3.1 Schematic showing the wetting mechanism of a liquid droplet on a solid surface in vapour or under another liquid. (a) a liquid droplet on an ideal flat lyophobic solid surface in vapour. a liquid droplet on a rough lyophobic solid surface in a wet-contact state (b) or a non-wet-contact state (c) in vapour. (d) a liquid droplet on a rough lyophilic solid surface in vapour. (e) a liquid droplet on a rough lyophobic solid surface in a transition state in vapour. (f) a droplet of liquid (L1) on a solid surface under another liquid (L2).
where θCa is the apparent liquid Ca in the Cassie mode, θCa-ideal is the liq- uid Ca for an ideal flat solid surface, fS is the area fraction of the solid on the surface, and rS is the roughness factor of the solid parts of the surface with which the liquid contact. For a lyophobic solid material, the grooves in rough surface trap vapour pockets and reduce the value of fS. as a result, θCa increases greatly (θCa > θCa-ideal) and the surface wettability is enhanced even to be superlyophobic in the vapour. and for a lyophilic solid material, the liquid will permeate inside the structural grooves (Figure 3.1d), leading to an enhanced lyophilic state (θCa > θCa-ideal).
the Cassie state reflects the lowest energy state in the open-vapour regime, also known as the metastable composite states, while the Wenzel state rep- resents the absolute minimum energy in a wetted lyophobic state.31,32 accord- ing to the difference between the Wenzel model and the Cassie model or eqn (3.2) and eqn (3.3), for a solid surface with same roughness, it should exhibit two distinct apparent liquid Cas. Generally, the liquid droplet in the Wenzel state always shows a stronger contact angle hysteresis than in the Cassie state.33 it has also been reported that the contact mode in the solid/liquid/vapour sys- tem will change and generate a transition model between the Cassie state and the Wenzel state when the liquid droplet is under pressure, vibration, impact or electrical field (Figure 3.1e).34–38 a threshold liquid Caθt (θt > 90°) between the Cassie state and the Wenzel state can be calculated by eqn (3.4).33,39
cos θt = (fS − 1)/(r − fS) (3.4) When θCa-ideal < θt, the as-trapped vapour pockets in the surface grooves are unstable, and the solid/vapour contact state will change to the Wenzel state.
When θCa-ideal > θt, the as-trapped vapour pockets are stable in the surface grooves, and the apparent Ca of a liquid droplet will agree with the Cassie mode.40
Besides the solid/liquid/vapour system, in the solid/liquid/liquid system, the liquid (named L1) wetting behaviour on the solid surface under another immiscible liquid (named L2) can be illustrated in Figure 3.1f and described by eqn (3.5).32,41
cos θL1L2 = (γSL2 − γSL1)/γL1L2 = (γL1a cos θL1 − γL2a cos θL2)/γL1L2 (3.5) where θL1L2 is the apparent L1 Ca on the solid surface under L2. θL1, θL2 are the apparent liquid 1 Ca and liquid 2 Ca on the solid surface in air, respec- tively. γSL2, γSL1, γL1L2, γL1a, γL2a and γL1L2 are the interfacial free energies of the solid/L2, solid/L1, L1/L2, L1/air, L2/air and L1/L2 interfaces, respectively. as for the solid/oil/water system, the oil Ca on a solid surface under water can be described by eqn (3.6).42
cos θoW = (γSW − γSo)/γoW = (γoa cos θo − γWa cos θW)/γoW (3.6) where θoW is the apparent oil Ca on the solid surface under water. θo, θW are the apparent oil Ca and water Ca on the solid surface in air, respectively.
γSW, γSo, γoW, γoa, γWa and γoW are the interfacial free energies of the solid/water, solid/oil, oil/water, oil/air, water/air and oil/water interfaces, respectively.
in summary, a superwetting (superlyophilic or superlyophobic) material can be constructed in combination of surface chemical composition (lyo- philic with a high surface energy or lyophobic with a low surface energy) and surface roughness.