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  • van der waals interactions between silica spheres and metallic thin films created by e-beam evaporation

    کلمات کلیدی :
    جزئیات بیشتر مقاله
    • تاریخ ارائه: 1392/07/24
    • تاریخ انتشار در تی پی بین: 1392/07/24
    • تعداد بازدید: 1054
    • تعداد پرسش و پاسخ ها: 0
    • شماره تماس دبیرخانه رویداد: -
     the long-range and adhesion forces between a molecularly smooth silica sphere and a relatively smooth thin metallic film have been measured by atomic force microscopy (afm) operating in contact mode. the majority of the dispersion force measurements have been done with root mean square (rms) roughness of more than 10 nm. this paper investigates lower roughness to prove or disprove theory of the van der waals force for irregularly distributed size of asperities. metallic films of cu, al, and ag were obtained by coating silicon wafers using an electron beam evaporation (e-beam) method. the e-beam deposition rates were within the range of 0.6–1.2 å/s at vacuum pressures between 2 × 10−6 and 3 × 10−6 mbar. the developed coatings had peak-to-peak distance ranges of 80–120 nm and 1–2 μm. long-range attractive forces were obtained from the extending portion of the force/distance curves measured between a silica particle attached to the afm cantilever and the metallic film surface. certain samples demonstrated an attractive force much larger than the theoretically calculated dispersion force, which we attribute to an electrostatic image force. for other samples, the extending curves were comparable with the theoretical dispersion forces, especially when the curves were shifted by a distance of 1.82 rms. adhesion forces were measured from the retracting portion of the force/distance curves. for peak-to-peak distance near 90 nm, films having an rms roughness of 0.5–3 nm displayed a force reduced by a factor of 3–40 from that predicted by theory. this effect of the surface roughness supports a dmt model for sample interaction at a single contact point after particle detachment. we also demonstrate the absence of any capillary forces under relative humidities up to 40%. it was explained by the thickness of wetting film smaller than the peaks height.

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