Sessile drop parameters

The usual experimental situation is that of a sessile drop and, as with the pendant drop, it is necessary to determine a shape parameter and some absolute length. Thus /3 may be determined by profile fitting, and Ze measured, where Ze is the distance from the plane at = 90 to the apex. If the drop rests with... 

The Bashforth-Adams tables provide an alternate way of evaluating 7 by observing the profile of a sessile drop of the liquid under investigation. If, after all, the drop profiles of Figure 6.15 can be drawn using 0 as a parameter, then it should also be possible to match an experimental drop profile with the (3 value that characterizes it. Equation (85) then relates 7 to 0 and other measurable quantities. This method is claimed to have an error of only 0.1%, but it is slow and tedious and hence not often the method of choice in practice. 

Fig. 4 Characteristic parameters of the processed photoresist layers as a function of the exposure dose a layer thickness, b RMS roughness (from AFM images), c isoelectric point (from zeta potential measurements), d contact angle (ADSA sessile drop). The dashed vertical line denotes the threshold dose...

The contact angle is by far the most often quoted characteristic to be derived from sessile drop experiments, but reference will be made later in this Section to other liquid parameters and particularly to the liquid surface energies, (tlv. While values for both 0 and sessile drop experiment it is often convenient to use differently sized drops. Small drops will assume the profiles of nearly spherical caps and this regularity assists the estimation of 0 values from their dimensions, while the gravitational... 

The parameter P is positive for oblate figures of revolution (for instance, a sessile drop or a meniscus in a capillary) but negative for prolate ones, like a pendent drop. In the absence of gravity P = 0 and the profile is spherical. 

The family of so-called bounded sessile configurations, that is, sessile drops and bubbles centered at the axis, belongs to the one-parameter subfamily... 

The parameter fi is positive for oblate figures of revolution, i.e. for the meniscus in a capillary, a sessile drop, and a bubble under a plate, and is negative for prolate figures, i.e. for a pendant drop or an adjacent bubble. Bashforth and Adams (1883) reported their results as tables. For more detailed information, see for example in Adamson (1967). 

The sessile drop method is similar to the pendant drop one [Sakai, 1965]. The same scheme is used, but in this case the droplet is resting on a plane surface immersed in the second component (see Figure 4.8) — Vj is calculated from analysis of the drop shape at equilibrium (characterized by the relative magnitude of the shape parameters, X and Z, dehned in Figure 4.8), knowing the densities of the polymeric huids at the temperature of measurement. 

The basic parameter for a liquid wetting a solid is the contact angle as defined in Fig. 1 for a sessile drop. It is formed by a tangent to the liquid at the air/liquid/solid line of contact and a line through the base of the liquid drop where it contacts the solid. Almost universally the angle reported is that measured through the drop as indicated in Fig. 1. 

When a drop of liquid rests on a solid supporting surface without any chemical reaction taking place, the actions of the various intermolecular forces present result in the drop assuming a particular shape. The shape strongly depends on the ratio of cohesive and repulsive forces. By studying the physical parameters of such a sessile drop, much information regarding the magnitude of surface forces maybe obtained. 

The sessile drop also involves a more dense polymer drop immersed in a less dense polymer matrix and allowed to settle on a flat surface (Fig. 5.34), where the parameters of h and r can be employed to determine p in the expression ... 

Fig. 2.1 Graphic vector representation of parameters in a sessile drop...

Fig. 2.5 (a) Dimensional parameters for a sessile drop, (b, c) plots of drop diameter and height as a function of drop volume for water on PFA, and (d, e, f) images of a 1, 50, and 2000 pL water sessile drop on PFA [inset photographs of the same drops at the same magnification] (Reproduced with permission from [15], Copyright 2010 The American Chemical Society)... 

Since the shape of a drop sitting (sessile) on a surface that it does not wet depends on the surface tension, we can measure the surface tension by making an accurate measurement of the parameters that characterize the shape of the drop. The profile of a drop is shown in Fig. 18.9. For large drops it can be shown that... 

It is usually called axisymmetric drop shape analysis.The interfacial tension and contact angles are determined from the shape of the axisymmetric menisci of both sessile and pendant drops. The employed strategy is to fit the shape of an experimental drop to the theoretical drop profile according to the Laplace equation, using surface tension as adjustable parameter. Details of the methodology together with a program to implement it can be found elsewhere. ... 

---