A molecular dynamics study of effective parameters on nano-droplet surface tension

Effectiveness of the Young-Laplace equation at nanoscale

Using molecular dynamics (MD) simulations, a new approach based on the behavior of pressurized water out of a nanopore (1.3–2.7 nm) in a flat plate is developed to calculate the relationship between the water surface curvature and the pressure difference across water surface. It is found that the water surface curvature is inversely proportional to the pressure difference across surface at nanoscale, and this relationship will be effective for different pore size, temperature, and even for electrolyte solutions. Based on the present results, we cannot only effectively determine the surface tension of water and the effects of temperature or electrolyte ions on the surface tension, but also show that the Young-Laplace (Y-L) equation is valid at nanoscale. In addition, the contact angle of water with the hydrophilic material can be further calculated by the relationship between the critical instable pressure of water surface (burst pressure) and nanopore size. Combining with the infiltration behavior of water into hydrophobic microchannels, the contact angle of water at nanoscale can be more accurately determined by measuring the critical pressure causing the instability of water surface, based on which the uncertainty of measuring the contact angle of water at nanoscale is highly reduced.

Characterization of the geometry of negative dielectrophoresis traps for particle immobilization in digital microfluidic platforms

This paper studies the effect of dielectrophoresis on particle manipulation and immobilization in digital microfluidic (DMF) devices. The dimensions of negative dielectrophoresis (nDEP) traps in the form of circular and square shapes are characterized using numerical and experimental approaches. These efforts will result in defining lifting and trapping zones, the ratio of which is shown to remain constant for trap sizes larger than 40 μm. As a result, a limiting constant K based on the ratio of the particle diameter to the trap size is introduced to identify the status of particle trapping prior to running numerical models or experiments. The results show that K must be less than 0.63 for trapping the particles on the nDEP traps. This study will also result in optimizing the trap size for single particle immobilization which is important for cell printing and growth applications.