Nanomaterials solvent evaporation ( drying ) process mechanism
Compared with conventional materials , nano- porous materials drying process not only to dehydration ( and / or solvent ) , but also to maintain nanoporous airgel nano pore structure ; nano-powder materials for the drying process , but also to prevent the particles from the between the reunion , and therefore its drying mechanism is much more complicated . Here, only the destruction of the gel structure scales collapse mechanism of agglomeration of nanoparticles and nanomaterials mechanism solvent evaporation ( drying ) process mechanism , etc. to be introduced.
    Generally, the gel solvent evaporation ( drying ) process goes through three small stage, the constant drying stages, the first stage and the second falling falling rate drying drying phase.
    In the constant rate drying stage, the drying rate and time and independent of sample thickness , gel volume contraction rate is equal to the liquid solvent evaporation rate , so the pores of the gel at this stage is always full of liquid. Shrinkage occurs in the initial stage , the skeleton of the gel is very soft . The capillary tube pressure of the system is small , with the contraction process proceeds, the gel skeleton becomes "hard ", and the shrinkage stress resistance is also enhanced, the capillary pressure is also increased. In the constant rate drying phase , the liquid in the gel pores is almost equal to the flow rate of the liquid surface of the evaporation rate. Hole liquid passes outward flow pattern achieved primarily through diffusion played a minimal role .
    With the evaporation of the solvent liquid , gel shrinking , the gel strength is small solid network backbone increasing until large enough to withstand the compression stress , the volume shrinkage is no longer able to maintain the liquid level in the condensate recess colloid surface , will enter the second stage of drying , the first falling rate drying phase. The critical point ( ie, the end of the constant rate drying phase , i.e. the starting point of the first drying stage deceleration > place, the radius of curvature of the meniscus gel pore radius is equal , if the contact angle is zero , then the maximum capillary pressure , the solid gel network backbone no contraction at the critical point, the evaporation rate begins to decrease , although the evaporation of the liquid phase has been entered within the gel , but still most of the evaporation of the liquid surface of the gel is not saturated by a thin layer of the inner wall of the hole continuously covered with a layer of liquid , the thin layer of liquid fluid passing hole may be provided - consecutive channels , therefore, in the first drying stage deceleration , the gel liquid hole pass flow mode is still largely based, accompanied by the diffusion of steam passing, in the first drying stage deceleration , the air - water interface has entered phase condensate limb , so there is always cracking of the gel was dried in the stage and a great relationship with the thickness of the gel .
    Deceleration in the first drying stage , the gel pores of the non- saturation region is covered by a thin layer of liquid , so the outer surface of the gel will dry out immediately , as long as the evaporation rate of liquid flow comparable , this state can always be maintained. But as for the evaporation process , the outer surface of the leading edge to the dry increasing distance , the pressure gradient is smaller, so that the liquid flow velocity, the outer surface of the distribution of the liquid slowly rendered discontinuous state, drying the gel into the third stage ( ie the second falling rate drying stage ) . In this stage, the gel phase is completely evaporated within the evaporation rate is no longer sensitive to external conditions , the gel pores near the outer surface of the liquid was not continuous state , the transmission fluid to flow and spread in two ways , but mainly by diffusion . Drying to a second drying stage deceleration , the total effect of the stress on the gel greatly eased, thus , the gel may expand slightly . Because skeleton undried side by side compression stress is larger than dry , the resulting stress difference may bending deformation of the gel . In this stage , the volume of the gel does not change , but the quality is gradually reduced. With this part of the liquid solvent evaporation , holes will further collapse occurs in varying degrees , in which the collapse of micropores disappear so that the original network structure shrunk to put one larger particles .