Microgroove-textured stainless steel surfaces are categorized in such a way that each group has approximately constant channel depth but having different pillar and channel width. The wetting characteristics, i.e., in terms of θ and D of the microgroove textured stainless steel surface have been investigated for the two different surface microgroove geometrical parameters:

Groove depth parameter,\(= \frac{d}{w}\)

Groove spacing parameter,\(= \frac{b}{w}\)

where, b – spacing between the two successive surface microgrooves, w – width of the microgroove, and d – depth of the microgroove.

Figure 3 and 4 illustrate the liquid drop configuration attained on the stainless steel surface induced by the size and spacing between the microgrooves present on it. While maintaining microgroove depth as constant and increasing the surface geometric parameters (d/w and b/w), the unidirectional groove-texture pattern on the surfaces induces anisotropic spreading properties, which in turn, leads to a difference in spreading behavior between the orthogonal and parallel to groove directions.

**Variation in Contact Diameter **

Figure 3 and 4 correlate qualitatively that the variation of surface structure parameters such as pillar width, channel width, and channel depth play a major role in the anisotropy, which is specific about the variation of wetting behavior between the parallel and orthogonal to the direction of the microgroove. A better quantitative understanding can be obtained by investigating the behavior of D|| and D⊥ with different surface microgroove geometrical parameters.

Figure 5 shows the variation of contact diameter with the geometrical parameters ξ = b/w for three different microgroove depths i.e., 30µm, 40µm, and 50 µm. The contact diameter, i.e., wetting liquid cap diameter decreases with increase in ξ in the direction parallel to groove for the three different microgroove depth under our investigations. At the same time, the variation of contact diameter is almost constant, even a slight increase is visible while increasing ξ for the wetting in the direction orthogonal to groove. Among the three different group of surfaces, the difference D⊥ and D|| increases with depth of the microgroove present on the surfaces. Increasing ξ, i.e., the possibility of microgroove structure having higher pillar width than the microgroove width causes reduction in preferential wetting by squeezing of the drop liquid in the direction of microgroove. This results in higher level of wetting by the drop liquid in the direction of groove than the wetting by the drop liquid in the direction orthogonal to groove and also the wetting diameter in the direction of groove decreases with increase in ξ. In addition to that, pinning of the three-phase contact line of drop liquid at the edge of the pillar initially results constant variation of contact diameter with ξ and further increase in ξ, i.e., pillar width pushes the pinning location further in the direction orthogonal to the microgroove which is responsible for slight increase in contact diameter.

Other than the geometrical surface parameter ξ, the geometrical surface parameter, (φ = d/w) provides information regarding the relative variation between the microgroove width and its depth on wetting diameter of the drop liquid attained on the microgroove-textured solid surfaces. Figure 6 illustrates the variation of D both in the direction parallel and orthogonal to microgroove with φ for three different group of surfaces with constant depth. As φ increases, the wetting diameter shows an increasing trend initially until certain value and then decreases in the direction both parallel and orthogonal to microgroove for the surface group with the microgroove depth of 30 µm and 60 µm. Comparing the variation of D among the surface groups with three different groove depths, the contact diameter is marginally less in the direction orthogonal to groove and also the contact diameter in the direction orthogonal to groove shows slight less magnitude while increasing the groove depth for the almost close by values of φ. In line with the reported variation of difference in contact diameter variation between the orthogonal and parallel to grooves for the geometrical surface parameter ξ, the difference in contact diameter increases while increasing the depth of the microgroove present on the surfaces. The trend in the variation of D⊥ and D|| with respect to φ and groove depth is mainly responsible to the pinning of TPCL by the edges of microgroove, followed by squeezing of the drop liquid flow into the microgrooves present in the surfaces.

**Variation in Static Contact Angle**

As illustrated in Fig. 1 about the schematic diagram of the liquid drop shape on the microgroove-textured solid surfaces, the variation of D is strongly connected with θ of liquid drop because the pinning of the three-phase contact point limits the variation of wetting of the drop liquid on the surfaces, which in turn, has detrimental effect on the θ of liquid drop. Figure 7 shows the variation of θ in the direction parallel to groove (θ||) and orthogonal to groove (θ⊥) with surface groove spacing parameter, ξ for three different group of the surfaces with differences in groove depth. As ξ increases, θ⊥ and θ|| increases until certain values and then show the decreasing trend for all three different group surfaces. Also, θ|| is lower than θ⊥ for all the values of ξ. In addition to this, the difference in variation between θ|| and θ⊥ increases with increase in the depth of the surface microgroove.

The variation in θ|| and θ⊥ with groove depth parameter, φ is shown in Fig. 8 for three different group of surfaces considered for the investigations. With increase in φ, both θ|| and θ⊥ decreases initially and then shows slight increasing trend and also θ|| is lower than θ⊥ for all the surfaces with different values of φ. In line with the variation of θ|| and θ⊥ with ξ, the difference between θ|| and θ⊥ increases with increase in the depth of the microgroove. The variation in θ|| and θ⊥ is governed by the pinning of TPCL by the edges of grooves in the direction orthogonal to groove and also preferential spreading along the passage of the microgroove.

Overall, an anomaly is visible in Fig. 5, 6, 7, and 8 while looking at the difference in variation between D|| and D⊥ and also variation between θ|| and θ⊥, the same differences decrease with increase ξ and φ for the surface groups with a depth of 30 µm, 40 µm, and 50 µm. One such a possibility is that the differences might be tending towards zero while increasing ξ and φ even more, i.e., towards an isotropic liquid drop shape configuration.

**Anisotropic Behavior of Liquid Drop Configuration**

The variation of D⊥, D||, θ|| and θ⊥ with surface microgroove parameters ξ and φ clearly indicate that the drop shape is clearly deviated from the spherical cap, i.e., approximately the shape of ellipsoidal cap. The geometrical shape variation of liquid drop shape configuration on the microgroove-textured solid surfaces can be characterized using anisotropic nature of the liquid drop shape configuration.

The anisotropy in contact diameter is quantified using

Eccentricity, \(\epsilon =\frac{{D}_{\left|\right|}}{{D}_{\perp }}\) ,

and its associated difference in θ between the orthogonal and parallel to the groove direction is expressed using

Wetting Anisotropy Δθ = θ⊥ -θ||

Figure 9a illustrate the variation of eccentricity (ε) with ξ and φ for three different group of surfaces with difference in groove depth. The magnitude of ε, that represents the shape of ellipsoidal cap formed by the drop liquid on the microgroove -textured surfaces decreases with increase in surface groove spacing parameter, ξ for the surface groups categorized with different microgroove depths. The magnitude of ε seems to be increasing while increasing the depth of microgroove present on the surfaces. Similarly, ε decreases its magnitude close to a level of drop shape configuration attained on the smooth surface while increasing the surface groove geometrical spacing parameter, φ. Also, the increase in depth of the microgroove results in higher value of ε. The variation in ε with respect to the surface microgroove parameters such as φ and ξ clearly indicates that the wetting by the drop liquid is dictated by the geometrical parameters of the microgroove such as microgroove width and depth and also spacing between the microgrooves.

There is a strong link between the wetting area formed by the drop liquid and its static contact angle of liquid – vapor interface with solid-liquid interface. Already the effect of surface microgroove geometrical parameters on the spreading diameter of the drop liquid has been discussed in detail in the preceding sections. At the same time, the θ of the liquid drop formed on the surface both in the direction orthogonal and parallel to the grooves have strongly been influenced by the surface groove geometrical parameters as shown in Figs. 7 & 8. Another anisotropic wetting behavior, wetting anisotropy (Δθ) (i.e., the difference in θ between the direction orthogonal and parallel to surface microgrooves) can be used to look at characteristic of wetting by the drop liquid on the groove-textured stainless steel surfaces. Figure 9 (b) explains the variation of Δθ with φ and ξ. When φ and ξ increases, the value of Δθ increases for the three different categories of surfaces with varying microgroove depth. Out of these three groups of surfaces, the surface group with higher microgroove depth attains the same value of Δθ that achieved by the surface group with lower microgroove depth at relatively higher value of φ. Overall, these phenomena have been occurred due to the preferential spreading of the drop liquid in the direction of groove by the squeezing effect induced through decrease in groove width and also the pinning of TPCL of the wetting liquid on the edges of pillar.