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In recent years, metamaterials and two-dimensional materials have revolutionized the fields of quantum mechanics and nanophotonics by introducing a new class of materials that can exhibit unique and exotic optical and thermal emission properties. This has been mainly driven by advances in nanofabrication and our ability to engineer surfaces with unique characteristics. Often, the surfaces of these materials are encountered by the presence of rarefied gases, where classical continuum description of the physical phenomena is no longer valid, and the non-continuum phenomena of gas-surface interactions (GSIs) dominate the energy transport from the surface to the gas particles. One example is the light-induced photophoretic levitation of nanostructured thin films and light-driven oscillators enabled by optical and photophoretic forces. Here, photophoretic or Knudsen forces (KFs) arise when a highly adsorbing material, immersed in a diluted gas, exchanges thermal energy via electromagnetic waves, thus inducing temperature inhomogeneities within the surface of the irradiated body. These temperature differences trigger non-equilibrium transport phenomena, where the gas particles and body collectively communicate with each other via GSIs. Here, the GSIs help transfer momentum from the thermally excited gas particles onto the surface, generating a temperature-driven force from the hot to the cold side of the body. On the other hand, if the temperature difference within the surface is negligible, Knudsen forces can still be induced by simply having different surface properties that modulate the efficacy of the GSIs. Although recent studies have demonstrated preliminary measurements of KFs for different gases and simple surfaces, there is little knowledge regarding how different surface characteristics affect the magnitude of the generated forces. Furthermore, there is no clear indication of to what extent the temperature gradient's magnitude along the surface affects the momentum exchange process between the impinging gas molecules. All these are important as KFs can play a major role in atomic force microscopy and photothermal-induced spectroscopy.

Hence, this work aims to perform an experimental investigation to unfold the nature of GSIs on the generation of Knudsen forces by measuring the force generated on smooth and functionalized surfaces for different temperature gradients, immersed in rarefied helium, nitrogen, and carbon dioxide gas. Here, surface functionalization will allow controlling the extent of GSIs by adjusting the net effect of specular/diffusive reflections in the form of the energy accommodation coefficients (EAC). As a preliminary step, we will follow the same procedure as our previous sub-continuum gas conduction investigations to characterize the GSIs by extracting the EACs for silicon and tungsten surfaces. Once the EACs are extracted, samples will be placed on a nanopositioning stage, equipped with a heating platform to heat samples to any desired temperature. Then samples will be brought into the proximity of a temperature-controlled suspended probe that helps detect the induced KFs via a laser interferometer. Then, adjusting the gap distance between the samples and the probe, in conjunction with the vacuum pressure, will allow us to measure the KFs forces for different transport regimes. The experimental results demonstrate a correlation between the measured forces and the value of the EAC extracted for all the samples. Furthermore, the magnitude of the force increases with the temperature gradient, as expected, due to an increase in kinetic energy. Lastly, it is shown that the forces generated by helium gas exceed those of nitrogen and carbon dioxide due to a high mismatch in the mean-free path. These results demonstrate that generated KFs can be altered depending on the type of gas-surface interaction occurring on the surface, paving the way to optimize this force by surface engineering.

Co-Author(s): Amun Jarzembski (Sandia National Laboratories), Mohammad Ghashami (University of Nebraska-Lincoln)

Greg Acosta

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