Experimental and theoretical investigations of evaporation of sessile water droplet on hydrophobic surfaces

Experiments of sessile water droplet evaporation on both polydimethylsiloxane (PDMS) and Teflon surfaces were conducted. All experiments begin with constant contact area mode (the initial contact angle is greater than 90°), switch to constant contact angle mode and end with mixed mode. Based on the assumptions of spherical droplet and uniform concentration gradient, theoretical analyses for both constant contact area and constant contact angle modes are made and theoretical solutions are derived accordingly, especially a theoretical solution of contact angle is presented first for CCR stage with any value of the initial contact angle. Moreover, comparisons between the theoretical solutions and experimental data of contact angle in CCR stage demonstrate the validity of the theoretical solution and it would help for a better understanding and application of water droplet on solid surfaces, which is quite often encountered in lab-on-a-chip, polymerase chain reaction (PCR) and other micro-fluidics devices.

Graphical abstractBoth CCR and CCA mode observed in experiments were theoretically analyzed and the theoretical solution of contact angle in the CCR stage was derived.Figure optionsDownload full-size imageDownload high-quality image (47 K)Download as PowerPoint slideHighlights► CCR, CCA and mixed modes were found in evaporation experiments of water droplet sessile on PDMS and Teflon surfaces. ► Theoretical solution of contact angle during CCR stage with any value of initial contact angle was derived. ► Time of CCR stage depends on variation of contact angles and proportional to D(c0 − c∞)/ρ and the square of contact radius. ► Variation rate of liquid-vapor area was on the order of D(c0 − c∞)/ρ, and the total evaporation time can be estimated.