measurement of contact angles The contact of 4 He crystals with walls: hysteresis of the contact angle S. Sasaki a, F. Caupin and S. Balibar Laboratoire de Physique Statistique de l'ENS (Paris, France) a - now at North Western Univ. (USA) Abstract: Abstract: We have measured the contact angle of the liquid-solid interface of helium 4 with different walls. It was known to be about 45° but we have found that is rather hysteretic with advancing angles of about 25° and receding angles of about 55 °. Although these angles are similar for copper, glass, and graphite substrates, the hysteresis seems to increase with roughness, as expected from the work of Rolley and Guthmann (Phys. Rev. Lett. 98, , 2007). This angle is an important parameter for the existence of liquid channels where grain boundaries meet walls. The hysteresis of the contact angle is known to increase with the substrate roughness (see, for example, the work of E. Rolley and C. Guthmann, PRL 98, , 2007). We see perhaps such a tendancy. The surface of our HOPG graphite was difficult to clean but, after 3 days pumping of the cell at 100°C, we could observe frequent nucleation of oriented crystals on either one or the other graphite piece (see photograph on the right). The contact angle on graphite is similar to that on glass or copper because the helium crystals are not oriented parallel to the graphite lattice. The value of the contact angle  c is important for the existence of liquid channels where grain boundaries meet walls. Indeed, these channels exist only if the contact angle  c  < , where  is half the dihedral angle of grooves appearing where GBs emerge at the liquid-solid interface. a square hole (11 x 11 mm 2 ) in a gold-plated copper plate (either 10 mm or 3 mm thick) 2 glass windows sealed with indium O-rings. the cell stands pressures up to at least 64 bar and can be cooled down to 35 mK. see Sasaki, Caupin and Balibar, submitted to J. Low Temp. Phys., July the experimental cells the shape of the liquid-solid interface depends on gravity and surface tension, as for an ordinary liquid surface. The contact angle si measured thanks to a fit with the Laplace equation near the wall: where x is the horizontal distance to the reference x 0, l c ~ 1 mm is the capillary length and z is the vertical height. Results: substrateadvancing anglereceding angle copper7 <  adv < 30°42 <  rec < 63° glass13 <  adv < 38° 40 <  rec < 66° graphite31 <  adv < 43° 44 <  rec < 62° During growth, the « advancing angle » is smaller than during melting (« receding angle »). We used either HOPG graphite which was difficult to clean properly since we coudl only warm it at 100°C under moderate vacuum (3 days pumping through the fill line) Discussion contact with a glass plate contact with a copper wall growing (16  m/s) melting (10  m/s) fits with Laplace eq. growing (60  m/s) melting (5  m/s) nucleation of an oriented helium crystal on HOPG graphite the contact line of a grain boundary (GB) with a wall is in reality a liquid channel with triangular cross section if the wall is sufficiently favorable to the liquid phase (if  c  <  crystal grain 1 crystal grain 2 liquid wall mixing chamber pressure gauge fill line (0.6 mm ID)