A valve assembly includes a tubular valve stem guide and a valve having a valve stem slidable in the bore of the valve stem guide. The peripheral wall has an upper edge defining an opening extending into the socket. The presence of the hydrolysable silane compound in the foam formulation leads to improved tensile, tear and elongation properties without an adverse effect on other important foam properties such as resiliency and hysteresis loss. A separation mechanism separates a first pair of forward and backward rotating rollers from each other. The power fluid chamber, the transfer chamber, and the product chamber are situated coaxially about the power fluid column. The optical modulator includes a substrate having an electro-optical effect and having a ridge, first and second grooves and first and second banks formed thereon, a Mach-Zehnder optical waveguide, an electrode, and first and second recesses formed at symmetrical positions with respect to the ridge on the first and second banks, respectively.