X-ray microscopy visualizes the indentation of colloids into soft elastic solids*

Soft matter at small length scales where the interfacial energy could be predominant often has curved interfaces to minimize the interfacial area, causing hardship in the optical-microscopy investigation. X-ray imaging seeing through the specimen in situ can overcome this hurdle, even requiring no specific sample treatment and environment. Using X-ray microscopy, we examine the adhesive contact between a silica sphere and a silicone elastomer substrate. Varying the particle sizes and Young's modulus of substrates, we confirm the crossover behavior between elastic and liquid-like substrates; The model combining the JKR theory and the surface stress of the substrate well describes the data. To our interest, the X-ray allows a direct measurement of the ridge height, in addition to the indentation depth and contact radius. The ridge height is linearly proportional to the indentation depth, not only in the elastic regime as expected by the JKR theory but also in the liquid-like regime.