Eletromolhamento em filmes de Langmuir-Blodgett de cera epicuticular

Abstract

Epicuticular wax (EW) is a natural compound found in the outermost part of the epidermis of plants, it may have amorphous or crystalline structure, being hydrophobic and with high electrical resistance. Due to these characteristics, it is proposed to use EW in electrowetting-on-dielectric (EWOD) system, a process in which the surface wettability of a dielectric material is changed by applying voltages. To prepare EW films, Langmuir and Langmuir-Blodgett (LB) techniques were used, which allow assembly with molecular scale control of the films. Langmuir films were characterized using measurements of surface pressure and surface potential, as well as Brewster angle microscopy (BAM). In these systems, the influence of the deposited dispersion volume and the subphase temperature on the film aggregation phases was investigated. LB films were characterized by atomic force microscopy (AFM) and electrowetting contact angle (ACE) measurements. For Langmuir films, it was noted that the maximum surface pressure decreases with the increase in the volume of EW dispersion deposited. In addition, three different aggregation regimes have been identified and associated with increasing the concentration of EW platelets as the trough area decreased. In temperature variation experiments for Langmuir films between 22 and 30 °C, surface pressure isotherms did not reveal a well-defined phase transition or collapse, while BAM images showed that, even before starting the compression procedure, there are EW platelets floating on the subphase. These platelets give rise to EW Langmuir films during compression. On the other hand, for higher temperatures (40 and 50 °C), platelets were not found and a well-defined phase transitions was observed with behavior similar to that found for typical fatty acids. The increase in surface potential during film compression suggested the presence of electrical dipoles, which are associated with functional chemical groups present in EW. BAM images of EW revealed that the barriers compression generates a homogeneous film in the condensed phase. The collapse occurs with appearance of grooves at 22 and 30 °C and by material overlapping at 40 and 50 °C. AFM images exhibit globular structures in LB films and demonstrate that film roughness decreases with increasing sub-phase temperature. Electrowetting experiments showed that films with 3 layers of EW exhibit an electrowetting process with an angle variation of ~ 20°.