A. Mas, M. Melnikov, and X. Tolsa.   A dual characterization of the C1 harmonic capacity and applications,   Duke Math. J., 153(1) (2010), 1-22. pdf
See also   Erratum to: “A dual characterization of the C1 harmonic capacity and applications”, Duke Math. J. 153(1) (2010), 1-22,   Duke Math. J., 157(2) (2011), 421-423. pdf