"成粘性的丝" can be translated as "forming viscous strings" or "forming sticky filaments" in English, depending on the specific context and the intended nuance.
"Viscous" emphasizes the thick, sticky, and slow-flowing nature of the substance.
"Sticky" focuses more on the adhesive property, indicating that the strings adhere to surfaces or each other.
Choose the phrase that best fits the context of your description.