It is caused by "frictional surface tension" or "contact angle hysteresis".
See
M. A. Tenan et al.,
"Friction in capillary systems,"
J. Applied Physics vol. 53, no. 10, pp.6687-6692, (1982).

At  9:50 98.7.16 +0200, Andreas Albrecht wrote:
>Dear colleagues
>We have a problem with reasoning and describing an initial
>pressure in a microfluidic body.
>Our aim is to reduce this initial pressure.
>In the attached file or at
>http://www.mikrotechnik-thueringen.de/pin7.htm you can find a
>schematic representation of our problem.
>We hope anyone can help.
>
>Regards,
>
>
>Meonic Micro System AG
>Haarbergstra=DFe 71
>D-99097 Erfurt
>e-Mail: [email protected]
>Mobil : +49 (0) 172 793 6205
>Phone : +49 (0) 361 420 5115
>Fax   : +49 (0) 361 420 5113

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e-mail: [email protected]