Localized material removal at tens of µm range is beneficial for studying chemical and mechanical properties of different samples, e.g., plants. To localize HIFU induced cavitation erosion at this level, the lateral width of the acoustic field must be comparable to the diameter of the desired removal area. Therefore, a suitable operating frequency of the transducer exceeds 20 MHz (corresponds to wavelength of 75 µm in water). As the thickness of the piezo element decreases with increasing frequency, the thinner piezo elements become ever more fragile. Therefore, the high frequency HIFU transducers are more difficult to construct and they withstand lower excitation amplitudes. To obviate these problems, we utilized third harmonics of in-house built transducers (7 MHz and 10 MHz) to decrease the cavitation erosion area. Test samples were ink-coated glass cover slides. We report measurement sweeps over transducer-to-sample distance and amplitude in relation to ink removal areas. The smallest removal areas were 50 µm in diameter by using the third harmonic of the 10 MHz transducer. The results propose that utilizing this method is beneficial for surface sampling purposes.
