Eliminating the surface location from soft matter contact mechanics measurements

The material properties of soft materials can be measured with rheometers and tensile testing instruments whenever there exist few limitations on sample volume, fixturing and general sample preparation, where samples often need to be prepared specifically to work with the hardware of a given instrument. By contrast, indentation methods are well suited for measuring material properties when sample preparation and geometry are highly constrained, as is the case with living cells, confluent cell layers, tissue samples, hydrogel coatings or soft objects with defined shapes like contact lenses. For example, indentation can be performed directly on cells grown in a Petri dish, without modifying typical cell culture protocols or materials. However, the low elastic modulus of these soft materials make it extremely difficult to determine when an indentation instrument first makes contact with a sample, which is critically important to know if material properties are to be determined with confidence. Here, we present an analysis method that eliminates the need to identify when an instrument makes contact with a sample. The method recasts the traditional force-displacement models of contact mechanics in terms of the first derivative of applied normal force with respect to indenter position, which automatically removes the unknown point of contact. This approach enables the selection of appropriate theoretical models for a given data-set and allows the measurement of sample material properties with the only fitting parameter being the elastic modulus.