tokens: of 2 mbar ambient pressure. The light intensity is ∼10 kW m −2 . The erosion of the dust starts milliseconds after the laser is turned on and is observed by a camera from the side. The measurements are based on the thermal creep of gas, which occurs if a temperature gradient is present along capillaries if the mean-free path of the ambient gas molecules is on the order of the size of the capillaries (Knudsen 1909 ). Pores in a dust bed act like capillaries. The basic setting is therefore a dust bed consisting of the sample material set in a vacuum chamber at a few mbar ambient pressure. A temperature gradient is generated by illuminating the surface of the sample. The resulting temperature profile is flat over the topmost dust layer but then decreases further down into the dust bed (de Beule et al. 2015 ). It leads to a gas flow by thermal creep from the cool deeper layers upwards toward the warm surface. The thermal creep flow itself is independent of a pressure and does not induce an overpressure. However, in the top layer without temperature gradient, a pressure is needed to maintain this flow (de Beule et al. 2015 ). Therefore, a pressure maximum evolves and the top layer of about 200 μ m is subject to an internal overpressure. If this stress is larger than the tensile strength, parts of a dust layer will be ejected. In recent work by de Beule et al. ( 2013 ), it was shown that the mass ejection rate N is inversely proportional to the cohesion per mass F and gravity g as In general, the idea is that a temperature gradient grows with time, the pressure takes some time to build up to the level of tensile strength and if a particle layer is ejected, the process starts all over again. This way, the time interval for particle ejections and therefore the ejection rates are coupled to the cohesive strength or tensile strength of the sample. The lower the cohesion, the smaller the time interval needed to build up the necessary pressure, and less cohesive samples result in higher ejection rates. In the case of this experimental setup and dust samples, the time span is on the order of milliseconds. de Beule et al. ( 2013 ) also showed that the brightness of the recorded frames of the erosion is a measure for the ejection rate. Hence, comparing the inverse of the brightness of the pictures in Figure 3 gives a relative value for the cohesion force (plus gravity) of the different dust samples. Figure 3. Overlay of ejected dust for different tempered samples. The overlay contains 2000 frames, which is a record time of 4 s. The laser intensity is ∼12 kW m −2 and the time of illumination is the same for each sample. Based on work by Muntz et al. ( 2002 ) and Sone Itakura ( 1990 ), de Beule et al. ( 2015 ) find for the overpressurewhere L x 1 marks the length (thickness) of dust layers with a temperature gradient and L x 2 is the active depth with constant temperature below the surface. On short timescales of milliseconds, the established temperature
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