||With the development of nanotechnology, lots of nanomaterials are being used to enhance mechanical strength, as well as thermal and electric conductivity. In recent years, graphitic nanomaterials have roused a great deal of interest in the fields of electronics, biological engineering, composite materials, photovoltaics, and energy storage due to their high carrier mobility, excellent optical transparency, and extraordinary mechanical and thermal properties. For such applications, inkjet printing has also become an important technology, as it is related to pattern formation in a drying drop containing specific solutes. This study investigates the surface properties of graphite colloids by electrochemical exfoliation and the behavior of pattern formation.|
Superhydrophilic graphite surfaces and water-dispersible graphite colloids are obtained by electrochemical exfoliation with hydrophobic graphite electrodes. Such counterintuitive characteristics are caused by partial oxidation and studied by examining both the graphite electrodes and the exfoliated particles after electrolysis. The degree of wettability of the graphite anode can be altered by the electrolytic current and time. After a sufficient time, the graphite anode becomes superhydrophilic and its hydrophobicity can be recovered by peeling with adhesive tape. This result reveals that anodic graphite is oxidized by oxygen bubbles but that the oxidation occurs only in the outer layers of the graphite sheet. The structure of this partially oxidized graphite may consist of a graphite core covered with an oxidized shell. The properties of the exfoliated colloids are also influenced by the pH of the electrolytic solution. As the pH increases, the extent of oxidation and the thickness of the oxidized shell decrease. These results reveal that the degree of oxidation of exfoliated nanoparticles can be manipulated by controlling pH.
A ring-shaped stain is frequently left on a substrate for a drying drop containing colloids due to contact line pinning and outward flow. However, different patterns are observed for drying drops that contain small-sized solutes or polymers on various hydrophilic substrates. Depending on the surface activity of the solutes and the contact angle hysteresis (CAH) of substrates, the pattern of evaporation stain will vary and may include concentrated stains, ring-like deposits, or combined structures. For small-sized surface-inactive solutes, the concentrated stain is formed on substrates with weak CAH, such as a copper sulfate solution on silica glass. On the contrary, a ring-like deposit is developed on substrates with strong CAH, such as a copper sulfate solution on graphite. For surface-active solutes, the wetting property can be significantly altered and the ring-like stain always appears, such as Brij-35 solution on acrylic glass. For a mixture of surface-active and surface-inactive solutes, a combined pattern of ring-like and concentrated stains can appear. Similar results are observed for various polymer solutions on a polycarbonate. Concentrated stains are formed for weak CAH, such as sodium polysulfonate (NaPSS), while ring-shaped patterns are developed for strong CAH, such as polyvinyl pyrrolidone (PVP). The stain pattern is determined by the competition between the time scales associated with contact line retreat and solute precipitation. The suppression of the coffee-ring effect can thus be acquired by controlling CAH.
Furthermore, depending on the surface-activity of the solutes, the extent of contact angle hysteresis (CAH) can vary with their concentration, accordingly altering the pattern of the evaporation stain. Four types of concentration-dependent CAH and evaporation stains have been identified for a water drop containing polymeric additives on polycarbonate. For polymers with surface-inactivity such as dextran, advancing and receding contact angles (a and r) are independent of solute concentrations and a concentrated stain is observed in the vicinity of the drop center after complete evaporation. For polymers with weak surface-activity such as polyethylene glycol (PEG), both a and r are decreased by the solute addition and the stain pattern varies with increasing PEG concentration, including a concentrated stain and a mountain-like island. For polymers with intermediate surface-activity such as NaPSS, a decreases slightly while r decreases significantly after a substantial addition of NaPSS and a ring-like stain pattern is observed. Moreover, the size of the ring stain can be controlled by the NaPSS concentration. For polymers with strong surface-activity such as PVP, a remains essentially constant but r is significantly lowered after a small addition of PVP and the typical ring-like stain is seen. For a mixture of surface-active and surface-inactive solutes such as PVP and CuSO4, the surface-inactive solutes can be deposited along the perimeter of the ring pattern due to contact line pinning induced by surface-active solute.
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