We have studied the spreading of a liquid 4He meniscus on two different cesium substrates with a ... more We have studied the spreading of a liquid 4He meniscus on two different cesium substrates with a controlled random roughness. The mean distance between the “mesa” defects is of the order of 20μm. In the case of strong pinning, we find that the roughness of the contact line scales as L1/2. Close to the wetting temperature, the liquid invades channels between the defects; this leads to a change in the scaling behavior of the roughness of the contact line.
How a liquid drop sits or moves depends on the physical and mechanical properties of the underlyi... more How a liquid drop sits or moves depends on the physical and mechanical properties of the underlying substrate. This can be seen in the hysteresis of the contact angle made by a drop on a solid, which is known to originate from surface heterogeneities, and in the slowing of droplet motion on deformable solids. Here, we show how a moving contact line can be used to characterize a molecularly thin polymer layer on a solid. We find that the hysteresis depends on the polymerization index and can be optimized to be vanishingly small (<0.07°). The mechanical properties are quantitatively deduced from the microscopic contact angle, which is proportional to the speed of the contact line and the Rouse relaxation time divided by the layer thickness, in agreement with theory. Our work opens the prospect of measuring the properties of functionalized interfaces in microfluidic and biomedical applications that are otherwise inaccessible.
ABSTRACT We have studied the spreading of helium-4 on cesium substrates with random disorder. Whe... more ABSTRACT We have studied the spreading of helium-4 on cesium substrates with random disorder. When the characteristic length scale of the disorder is of the order of 20 micrometers, we have been able to measure the roughness exponents of the contact line, and found that they are in agreement with theoretical predictions. The temperature dependence of the roughness is also consistent with the theory. However, the same model predicts an hysteresis much smaller than the measured one. Better cesium substrates with optical quality yield a very straight contact line but the same large hysteresis. This hysteresis is presumably due to the pinning of the line on microscopic defects of the evaporated cesium film. The velocity of the contact line increases exponentially with the applied force, suggesting that the depinning is thermally activated.
A rich variety of phenomena are found when studying the growth of helium crystals. In the present... more A rich variety of phenomena are found when studying the growth of helium crystals. In the present review, a particular emphasis is given on three particular topics. We first compare rough surfaces in helium 4 and in helium 3, and show how very different thermal excitations lead to very different growth processes. We then summarize the results obtained on the
dc micromotors have been used to rotate helium 4 crystals in a pressurized cell (25 bars) at very... more dc micromotors have been used to rotate helium 4 crystals in a pressurized cell (25 bars) at very low temperature (down to 30 mK). After slight modifications of the motors and optimization of bearings and transmission design, rotations of ±6° were obtained with an accuracy of ±0.01° and a dissipation of 20 μW.
We have studied the spreading of a liquid 4He meniscus on two different cesium substrates with a ... more We have studied the spreading of a liquid 4He meniscus on two different cesium substrates with a controlled random roughness. The mean distance between the “mesa” defects is of the order of 20μm. In the case of strong pinning, we find that the roughness of the contact line scales as L1/2. Close to the wetting temperature, the liquid invades channels between the defects; this leads to a change in the scaling behavior of the roughness of the contact line.
How a liquid drop sits or moves depends on the physical and mechanical properties of the underlyi... more How a liquid drop sits or moves depends on the physical and mechanical properties of the underlying substrate. This can be seen in the hysteresis of the contact angle made by a drop on a solid, which is known to originate from surface heterogeneities, and in the slowing of droplet motion on deformable solids. Here, we show how a moving contact line can be used to characterize a molecularly thin polymer layer on a solid. We find that the hysteresis depends on the polymerization index and can be optimized to be vanishingly small (<0.07°). The mechanical properties are quantitatively deduced from the microscopic contact angle, which is proportional to the speed of the contact line and the Rouse relaxation time divided by the layer thickness, in agreement with theory. Our work opens the prospect of measuring the properties of functionalized interfaces in microfluidic and biomedical applications that are otherwise inaccessible.
ABSTRACT We have studied the spreading of helium-4 on cesium substrates with random disorder. Whe... more ABSTRACT We have studied the spreading of helium-4 on cesium substrates with random disorder. When the characteristic length scale of the disorder is of the order of 20 micrometers, we have been able to measure the roughness exponents of the contact line, and found that they are in agreement with theoretical predictions. The temperature dependence of the roughness is also consistent with the theory. However, the same model predicts an hysteresis much smaller than the measured one. Better cesium substrates with optical quality yield a very straight contact line but the same large hysteresis. This hysteresis is presumably due to the pinning of the line on microscopic defects of the evaporated cesium film. The velocity of the contact line increases exponentially with the applied force, suggesting that the depinning is thermally activated.
A rich variety of phenomena are found when studying the growth of helium crystals. In the present... more A rich variety of phenomena are found when studying the growth of helium crystals. In the present review, a particular emphasis is given on three particular topics. We first compare rough surfaces in helium 4 and in helium 3, and show how very different thermal excitations lead to very different growth processes. We then summarize the results obtained on the
dc micromotors have been used to rotate helium 4 crystals in a pressurized cell (25 bars) at very... more dc micromotors have been used to rotate helium 4 crystals in a pressurized cell (25 bars) at very low temperature (down to 30 mK). After slight modifications of the motors and optimization of bearings and transmission design, rotations of ±6° were obtained with an accuracy of ±0.01° and a dissipation of 20 μW.
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