(Top) Latex rubber samples at various strains. The strain, ε, is determined from the size of the stamped pattern. (Bottom) Strain is applied by using an O-ring to isotropically stretch the latex over a hollow copper cylinder.
Schematic of the experimental apparatus used to controllably contact a PTFE sphere with a latex rubber sheet of applied strain ε, and measure the resulting surface potential on the latex. The surface potential is measured in real time by an electrostatic voltmeter probe positioned at the backside of the latex sheet. The linear feed moves the PTFE sphere towards and away from the latex surface at a constant rate. The entire setup is enclosed inside a Faraday cage to block external electric fields, and placed inside a glove box to control the relative humidity.
Surface potential measurements for a single contact between a PTFE sphere and latex rubber sheet (ε = 0%). (Top) The approach, contact, and separation of the PTFE sphere and latex are schematically illustrated. (Middle) The distance, d, between the PTFE sphere and the latex is shown as a function of time. (Bottom) The surface potential measured by the probe under the latex surface is shown. The surface potential was measured in real time every second.
Surface potential measurements for repeated contacts between a PTFE sphere and latex rubber sheet (ε = 0%). (Top) The distance, d, between the PTFE sphere and the latex as a function of time; in this case, 8 contacts were performed. (Bottom) The surface potential measured in real time by the probe under the latex sheet. The data indicate that the latex charges negatively and the PTFE charges positively, with the magnitude of the charging increasing at each contact.
Surface potential measurements for repeated contacts between the PTFE sphere and the latex sheets of varying strain, ε. (Top) The distance, d, between the PTFE sphere and the latex surface as a function of time; all samples were contacted 8 times. (Bottom) The surface potential measured in real time by a probe under the latex sheet; the strain on the latex for each set of data is indicated. The surface potential shifts significantly as the strain increases, with high strain causing a reversal of the direction of charge transfer.
Summary of surface potential measurements for latex sheets as a function of strain, after 7 contacts with a PTFE sphere. The data represent the results of 28 experimental trials.
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