A Cassie-Like Law Using Triple Phase Boundary Line Fractions for Faceted Droplets on Chemically Heterogeneous Surfaces

Abstract

We present experimental contact angle data for surfaces, which were surface-engineered with a hydrophobic micropattern of hexagonal geometry. The chemically heterogeneous surface of the same hexagonal pattern of defects resulted in faceted droplets of hexagonal shape. When measuring the advancing contact angles with a viewing position aligned parallel to rows of defects, we found that an area averaged Cassie-law failed in describing the data. By replacing the area fractions by line fractions of the triple phase boundary line segments in the Cassie equation, we found excellent agreement with data.

Wetting transitions on biomimetic surfaces

“…It should be mentioned that dependence of the APCA on equilibrium contact angles on the hierarchical surfaces is weak when compared with those on single-scale surfaces (Bormashenko et al 2008b). Modifications of the Cassie-Baxter equation that take into account the peculiarities of the complicated topography of biomimetic surfaces were reported recently (Nosonovsky & Bhushan 2008;Choi et al 2009;Larsen & Taboryski 2009). …”

Understanding the intrinsic water wettability of graphite

“…Their 2009 report investigated the wetting mechanism using a patterned hydrophilic substrate with hydrophobic defects (Figure 18). 253 This situation is opposite that encountered on fresh graphite (mildly hydrophilic surface with defects that are more hydrophilic) although the wetting mechanism remains similar for both situations. A quick glance at Figure 18a shows that the water contact line is irregular and not symmetrical.…”

“…Nearly all of the most recent work done to elucidate the causes of hysteresis have sought to explain phenomena surrounding superhydrophobic surfaces; therefore, they use micropatterned surfaces with a repetitive pillar array. 251,[253][254][255][256] The pillars are typically on the micron scale and range from a few to several microns in height, width, and spacing. When a water drop is placed on this micropatterned surface, it will interact with a composite surface of (1) the pillars and (2) Larsen and Taboryski studied the behaviour of hysteresis on chemically heterogeneous surfaces and showed that the contact line is distorted by defects and behaviour of the advancing and receding contact angle is dependent on surface wetting compared to defect wetting.…”

“…I.e., a slightly chemically heterogeneous will behave more like a homogeneous surface than a significantly chemically heterogeneous surface. 251,253 In relation to the model, graphite can be considered to be mildly hydrophilic with defects that are more hydrophilic. Thus it can be modeled as in Figure 19 The size of an individual defect is proportionally related to the surface defect density (i.e., larger defects increase defect density and decrease basal surface).…”

“…The experimentally observed WCA can now be defined as the thermodynamic equilibrium of water, defects, and substrate along the contact line. 251,253,262 Adding liquid to the water drop will cause WCA to increase while maintaining a constant diameter. This is due to pinning of the contact line as shown in in Figure 20b.…”