Abstract
Interface conditions are an important property that can affect the drag of fluid flow. For surfaces with different oleophobicity, the boundary slip at the solid–oil interface is mostly larger than that at the solid–water interface. Roughness is a key factor for the wettability of superoleophilic/superoleophobic surfaces, and it has been found to affect the effective value of slip length in measurements. Moreover, there are no studies on the effect of roughness on slip at interfaces between oil and superoleophilic/superoleophobic surfaces. A theoretical description of the real surface roughness is yet to be found. Results show that the effective slip length is negative and decreases with an increasing root mean squared (RMS) roughness of surfaces, as the increasing roughness enhances the area with discontinuous slip at the solid–liquid interface. The underlying mechanisms are analyzed. The amplitude parameters of surface roughness could significantly inhibit the degree of boundary slip on both superoleophilic surfaces in Wenzel state and superoleophobic surfaces in Cassie state immersed in oil. The oleic systems were likely to enhance boundary slip and resulted in a corresponding reduction in drag with decreasing roughness on the solid–oil interfaces.
Highlights
In micro/nanofluidic systems, the increasing surface to volume ratio leads to unignorable fluid drag at the solid–liquid interface
As the slip length b is obtained through the average hydrodynamic force Fhydro and its absolute value equals the height of the area, the sample Rz roughness root mean squared (RMS) roughness hexadecane θAa θRa ethylene glycol θAa θRa aθA is the advancing contact angle, while θB is the receding contact angle
The effect of surface RMS roughness on the interface conditions was studied by using laser confocal scanning microscopy and atomic force microscopy (AFM)
Summary
In micro/nanofluidic systems, the increasing surface to volume ratio leads to unignorable fluid drag at the solid–liquid interface. Reference surface of superoleophilic and superoleophobic samples were defined and the effective slip lengths were obtained.
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