Abstract:

When predicting the effectiveness of particles as droplet stabilizers, one often assumes that the contact angle (advancing or receding) is a property of the materials. We find that, by contrast, the receding angle is sensitive to the interface shape, while the advancing angle remains constant. In particular, we find that the receding angle decreases substantially with increasing anisotropy of the interface shape (parameterized by the deviatoric curvature). Measurements we made with carefully cleaned glass spheres functionalized with PDMS; angles were measured by image analysis and control experiments were done.

Suggested Citation:

Nesrin Senbil. 2015. Wetting and Pickering Emulsions: measured receding contact angles on spherical particles as a function of interface shape.. Distributed by: GRIIDC, Harte Research Institute, Texas A&M University–Corpus Christi. doi:10.7266/N71J97QZ

Purpose:

Our goal was to understand the adsorption of solid particles at liquid interfaces. The key to adsorption is the angle between the solid surface and the interface, known as the contact angle. We investigate how this angle depends on whether the contact is advancing or receding, and on the shape of the interface.

Data Parameters and Units:

The primary data are contact angles (degrees), and a mesaure of the interface shape known as deviatoric curvature (inverse millimeters).

Methods:

In our experiments, we measured contact angle of a sphere at air/water interface. The sphere was 3.2mm in diameter, made of borosilicate glass (Winsted Precision Ball Company) and coated with PDMS (Gelest cat no. DMS-T22), attached to a rigid rod and pushed into the interface, and then pulled back up. As the sphere is pushed into the water, the water/air interface advances across the sphere and the angle between the 3-phase is the apparent advancing angle. In the reverse (lowering) process, once the interface totally recedes from the sphere, a consistent contact angle (the apparent receding angle) is observed. Contact angle of the sphere was measure at both the left and right sides of the sphere in the receding limit. For each measurement, the sphere was displaced into the water by a small amount, then le ft for 2 minutes to reach steady state. As seen from the included data, the apparent receding angle decreases as the Deviatoric Curvature (anisotropy of the interface) increases. (D is defined as the ½ the difference between the two principal radii of curvature and is measured from our images. It is a measure of anisotropy of curvature. ) The way we have increased the deviatoric curvature is that we placed a secondary sphere astride the interface to make a saddle-like shape. In the following data, D=0 for initially flat interface, and not zero for other data points. We see a similar change in receding angle when we use an initially cylindrical interface.

Instruments:

Digital camera, white-light lamps.

Error Analysis:

Measurements were repeated and standard methods used to find uncertainty.

Provenance and Historical References:

Further information (including subsequent experiments) may be found in R1.x141.066:0062

Individual Files: