(525j) Novel Methodology to Measure the Contact Angle Between a Liquid and a Powder Bed By Using Droplet Penetration Technique

Determination of the contact angle between a liquid and a solid porous material is not trivial but crucial for many engineering applications including pharmaceutical, agricultural, printing, etc. and it can be also used to characterize the solid particle surface energy by measuring the contact angle with multiple liquids. In this work, we use the droplet penetration technique which has the advantages over the more traditional â??Washburnâ? column technique in that the amount of material necessary to perform the experiment and the time it takes are orders of magnitude smaller.

The novel model is based on the comparison between the non-dimensional volume of liquid penetrated at the same non-dimensional times when using two different liquids: the test liquid (water or Diiodomethane) and a reference liquid (PDMS), which completely wets the granular material. This approach takes advantage of the fact that under certain assumptions, easily verified in most cases, the non-dimensional dynamics of a droplet penetrating into a porous material doesnâ??t depend on any length scale except for the radius of the area of contact between the droplet and the material. As a result, when this scale is constant, the non-dimensional dynamics is universal. The advantage of this approach is that the resulting contact angle does not depend on the actual sorption dynamics: whether unidimensional as usually assumed, with the front propagating only in the axial direction, or bi-dimensional, when there is also front propagation in the radial direction. As long as a large part of the penetration process occurs at constant contact area, condition that is amply confirmed in our systems, we found a simple analytical expression to determine the contact angle based only on computing the ratio between non-dimensional penetration times of the test and reference liquids.

We show the universality of this approach by performing experiments with droplet of different size, different liquids and different powders. We also perform the analysis for different times (and penetrated volume) and compare the results with those obtained using the more traditional approach "Washburn" column technique.