I get how mutual capacitance touch screens work, by changing the dielectric around the pairs of electrode and "stealing" electric field lines. Connection to earth of the object (a finger usually) isn't required.
What I don't get with self capacitance measurement is that the finger-electrode pair forms a capacitor connected to earth through the capacitance between the human body and earth. How could the smartphone measure that capacitance without itself being connected to earth? How could it load this newly formed capacitor in any way without "completing the circuit"?
Is that because the smartphone is usually also connected to ground through capacitive coupling (as is the human body)? It troubles me because it's never mentionned in any of the explanation I've seen about this technology.
My preferred ad hoc explanation of this phenomena is that the circuit is closed through capacitive coupling between the human body and smartphone ground, thus eliminating the need of earth. But I'm not really convinced since the capacitance of the coupling between the human body and the smartphone is extremely variable and largely unknown.