Evaporation of Femto Droplets

Besides numerous applications in surface nanopatterning, the tools presented in this paper can also be used for more fundamental studies of wetting processes at submicron scale. To illustrate this point, the evaporation of droplets produced by NADIS with volumes smaller than the femtoliter was performed. When manipulating liquids at submicron scale, evaporation becomes an important issue. It is therefore essential to study experimentally the evaporation of submicron droplets since most of the previous comprehensive studies of evaporation have been limited to microliter droplets (i.e., diameters in the millimeter range).  

Droplets with diameters in the micron range are obtained using a NADIS tip with a 300 nm aperture and a highly hydrophilic outer wall (Fig. 12.17a). The NADIS tip is aligned with the QBR using a nanopositioning table incorporated in APM.  

During evaporation, the resonance frequency of the QBR is monitored (Fig. 12.17b inset). A calibration curve obtained with calibrated silica beads is used to convert the resonance frequency shifts into mass changes so that the temporal evolution of the droplets mass is determined down to around 10 fg (10 aL). 

Time evolution of the mass of evaporating glycerol droplets with initial volumes ranging from 0.2 fL to 20 fL are shown in Fig. 12.17b. The droplet mass decreases nonlinearly, with a slowing-down of the 

QBR resonator before deposition (Inset zoom on the QBR after deposition of a 2 pm droplet], (b] Evaporation curves of droplets with volumes ranging between 0.15 and 15 fl in an vs. t representation (inset phase resonance curves before deposition [red] and during the evaporation process after deposition]. 

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