Microdrop evaporation

Keywords Atomic force microscopy Evaporation law Microdrop evaporation Micromechanical cantilevers Surface tension Vaporization heat Young s equation... 

Time of wetness (TOW), considered as the time during which the corrosion process occurs, is an important parameter to study the atmospheric corrosion of metals. According to ISO-9223 standard, TOW is approximately the time when relative humidity exceeds 80% and temperature is higher than 0°C. No upper limit for temperature is established. In tropical climates, when temperature reaches values over 25°C, evaporation of water plays an important role and the possibility to establish an upper limit respecting temperature should be analyzed. The concept of TOW assumes the presence on the metallic surface of a water layer however, there are recent reports about the formation of water microdrops during the initial periods of atmospheric corrosion, showing that the idea of the presence of thin uniform water layers is not completely in agreement with the real situation in some cases (particularly indoor exposures). 

Einally, Ma and Cantwell" " developed liquid-liquid-liquid microextraction (LLLME) to achieve preconcentration and purification for polar analytes without the need for both solvent evaporation and analyte desorption. The compounds were extracted from aqueous samples (donor phase) into an organic phase, layered on the donor phase, then back extracted to the receiving phase, and suspended in the organic phase. After extraction, the microdrop was injected into the HPLC system directly for analysis " (Table 11.3). 

Microcapsule shells formed in pol Tner solutions or suspensions can be solidified by drying. A non-volatile inhibitor is dispersed in an easily volatile liquid, which is the polymer carrier. This mixture is then sprayed in a heated chamber and the liquid evaporates quickly. As a result, film shells are formed on the microparticles or microdrops of the inhibitor during evaporation. 

Here 1 show how the use of atomic force microscope (AFM) cantilevers as sensitive stress, mass, and temperature sensors can be employed to monitor the evaporation of microdrops of water. Starting drop diameters are always below 100 p,m. The foremost interest lies in exploring the last stages of the evaporation process. 

Summarizing, one can say that when a small, non evaporating microdrop is sitting on a surface and forms a finite contact angle with it, the macroscopic picture is ... 

We examined the evaporation of microdrops of water with different initial volumina on a silicon surface coated with a 30 nm thin fluoropolymer film (perfluoro-1,3-dimethylcyclohexane). The initial contact angle was = 90° and remained constant for more than half of the evaporation time. During the experiments, the temperature (T = 25 °C) and the relative humidity (RH 99%) were constant. We used a video microscope to track the dimensions of the evaporating drop from the side [23,30]. 

A typical evaporation curve of a water microdrop on a silicon cantilever, acquired at NPT and RH 30%, is a plot of the inclination of the cantilever versus time (Fig. 4A). At the same time, the contact angle and the contact radius a are recorded with a video microscope from the side (Fig. 4B and C). The water microdrop is deposited onto the cantilever at t = 0 with the inkjet device, and immediately starts evaporating. The evaporation is over after 0.6 s, as the cantilever s inclination returns to its initial value. In the contact angle and contact radius curves, the black lines are simply guides to the eye. They show that two evaporation modi take place at the beginning, the drop evaporates in the constant-contact-radius (CCR) mode, and after 0.3 s both, and a, decrease linearly with time. Plots of V and of versus time demonstrate the agreement with the evaporation law derived in Eq. 6 for a drop evaporating in non-saturated vapor (Fig. 4D). 

Fig. 9 A Simultaneously acquired inclination full circles) and resonance frequency hollow diamonds) of a cantilever versus time upon evaporation of a water microdrop on a hydrophobized silicon cantilever. F 25 °C, RH 30%. Drop data a = 33 pm, 0 = 80°, Yi = 0.072 N/m, mo = 60 ng. Cantilever data Iq - 500 pm, w = 90 pm, d = 2.l pm. B Drop mass versus time from frequency hollow diamonds) and video solid triangles) data, and linear fit solid line)...

Fig. 10 Experimental solid line), simulated hollow squares), and difference hollow triangles) inclination of silicon cantilevers versus time upon evaporation of water microdrops. T 25 °C, RH 30%. Drop data various initial volumes, contact radii, and contact angles Pl = 0.072 N/m. Cantilever data Zq = 750 pm, w = 90 pm, d = l.S pm, u b = 1-5 pm, and dc = 1 7 pm gold layer thickness = 30nm Young s moduli Esi = 180 GPa, au = 78 GPa Poisson s ratios vsi - 0.26, uau = 0.44...

Procedure, b) A spot plate or a micro crucible is used, a drop of the test solution is mixed with a microdrop of 1 % alcoholic fluorescein solution and a microdrop of a 10 1 mixture of glacial acetic acid and 30 % hydrogen peroxide, and evaporated to dryness. A pink to red color appears. A blank test on a drop of water is advisable when small amounts of bromine are suspected. 

---