Dry dust is the easiest dirt for the camera to shake off but often it is not dry due to humidity or the source of the dust. The natural enemy of sensors is pollen and spores which is small, airborne and sticky. It can be as small as 6micrometers in diameter and has a coating of waxes and proteins that allows it to stick to plant stamens on contact. Same with nice landing surfaces like AA filters. The smallest grains are not visible on a sensor but some of the more common pollen sources around houses and development are just large enough to cause problems. The photos shown above look a lot more spherical than a liquid, since the greatest light absorption is in the center, where a liquid would be the least in the center and furthest boundaries. I have no idea what it is, there are literally a million possibilities and until someone does a an analysis of the composition of the specks it is all a guess. The least likely causes are those that the forums jump to conclusions about because of how firmly their belief is that it is an assembly defect the less plausible it becomes. A look on any surface with a microscope will reveal another world unseen and unrecognized in daily life. That world is filled with microscopic particles coating every surface. What is amazing is how little dirt appears on sensors when everything else is covered with it. Some dirt, in daily visible life, is large and can be seen, it is not unusual to have to clean lenses, both outside and inside tightly screwed on filters, I do it routinely before every shoot of any importance, because it is needed routinely. If dirt accumulates enough to be detrimental for sensors, say, on 6 months intervals, that really is pretty good and cleaner than any other surface involved in ones life.