A testing device for determining the presence of substances on a surface , the testing device comprising: an imaging sensor positioned at a known vantage point determined by a position determining component through measurement of distance from and angle to a surface ; a liquid dispenser to deposit one or more liquid drops on the surface ; a data generator to: after a first time interval, detect via the imaging sensor a geometric characteristic of the liquid drop that is related to a contact angle of the liquid drop to the surface corresponding to the first time interval; after a second time interval, detect via the imaging detector the geometric characteristic of the liquid drop that is related to a second contact angle of the liquid drop to the surface corresponding to the second time interval; determine the presence of a substance on the surface by comparing the first and second contact angle; wherein said substance is capable of interacting with the liquid forming the drop through dissolution or some other chemical interaction, said dissolution or some other chemical interaction causing a change in the surface tension of the liquid and/or in the surface energy of the surface thereby causing a change between the first and second contact angles. an imaging sensor positioned at a known vantage point determined by a position determining component through measurement of distance from and angle to a surface ; a liquid dispenser to deposit one or more liquid drops on the surface ; a data generator to: after a first time interval, detect via the imaging sensor a geometric characteristic of the liquid drop that is related to a contact angle of the liquid drop to the surface corresponding to the first time interval; after a second time interval, detect via the imaging detector the geometric characteristic of the liquid drop that is related to a second contact angle of the liquid drop to the surface corresponding to the second time interval; determine the presence of a substance on the surface by comparing the first and second contact angle; wherein said substance is capable of interacting with the liquid forming the drop through dissolution or some other chemical interaction, said dissolution or some other chemical interaction causing a change in the surface tension of the liquid and/or in the surface energy of the surface thereby causing a change between the first and second contact angles. after a first time interval, detect via the imaging sensor a geometric characteristic of the liquid drop that is related to a contact angle of the liquid drop to the surface corresponding to the first time interval; after a second time interval, detect via the imaging detector the geometric characteristic of the liquid drop that is related to a second contact angle of the liquid drop to the surface corresponding to the second time interval; determine the presence of a substance on the surface by comparing the first and second contact angle; wherein said substance is capable of interacting with the liquid forming the drop through dissolution or some other chemical interaction, said dissolution or some other chemical interaction causing a change in the surface tension of the liquid and/or in the surface energy of the surface thereby causing a change between the first and second contact angles. <EOS>