Mixidity? Quantitative property of mixed subjects

Solution 1:

Unable to suggest the best word. But right word for mixability is miscibility.

Exploring the root of word emulsion might be helpful.

Also, in physical chemistry there is a possibility of finding the term you are looking for.

Solution 2:

What you're describing is the state of a heterogenous mix, or a mixture where two different samples may have elements in different proportion (ThoughtCo). If it were perfectly mixed, it would be homogenous.

So what you're investigating is the heterogeneity of a given mixture (Merriam-Webster):

: the quality or state of consisting of dissimilar or diverse elements : the quality or state of being heterogeneous

This can be quantified, as Gy's sampling theory does (Wikipedia), in heterogeneity tests with various applications. Here's one in geology (Geostats Pty Ltd):

The heterogeneity test uses the principles of Pierre M. Gy's Sampling Theory.

The test work establishes appropriate sampling protocols for particular ores and allows for the design of sample flow sheets from the field through to sample preparation and assaying. Errors at each stage of sample size diminution can be calculated.