Casimir expulsion of periodic configurations
  There is the possibility in principle that the noncompensated Casimir force exists in open nanosized metal cavities arranged in the form of periodic structures. It is found that when trapezoid cavities are strictly periodic all the Casimir expulsion forces are completely compensated. However, when the distance of the gap is formed between the cavities, in the periodic configuration a noncompensated expulsion force proportional to the number of cavities appears. There are such effective parameters of the periodic configuration (the angles of the opening of cavities, their lengths and the relationships between them) which lead to the appearance of a maximum of expulsion forces per unit of structure length. 
Evgeny G. Fateev
quant-ph