Laser-induced forward transfer of liquids: Study of the droplet ejection process

Abstract

Laser-induced forward transfer ͑LIFT͒ is a laser direct-write technique that offers the possibility of printing patterns with a high spatial resolution from a wide range of materials in a solid or liquid state, such as conductors, dielectrics, and biomolecules in solution. This versatility has made LIFT a very promising alternative to lithography-based processes for the rapid prototyping of biomolecule microarrays. Here, we study the transfer process through the LIFT of droplets of a solution suitable for microarray preparation. The laser pulse energy and beam size were systematically varied, and the effect on the transferred droplets was evaluated. Controlled transfers in which the deposited droplets displayed optimal features could be obtained by varying these parameters. In addition, the transferred droplet volume displayed a linear dependence on the laser pulse energy. This dependence allowed determining a threshold energy density value, independent of the laser focusing conditions, which acted as necessary conditions for the transfer to occur. The corresponding sufficient condition was given by a different total energy threshold for each laser beam dimension. The threshold energy density was found to be the dimensional parameter that determined the amount of the transferred liquid per laser pulse, and there was no substantial loss of material due to liquid vaporization during the transfer.

Time-resolved imaging of the laser forward transfer of liquids

“…1. As it has already been reported in previous works, 9 circular and well-defined droplets are obtained. However, at fluences out of this range different situations occur: at high fluences irregular droplets are deposited, and at even higher fluences only splashing is observed; on the other hand, at low fluences no material is transferred.…”

“…2͒ presents an upward trend with a clear linear increase, as it has also been observed in prior works. 9,15,20 The droplet diameter, which sets the resolution of the technique, plotted versus the laser fluence ͑Fig. 2͒ also presents an upward trend, but the increase is not linear.…”

“…7,8 The translation of both donor and receptor with respect to the laser beam allows printing micropatterns of any predefined two-dimensional geometry. The technique has also a high degree of spatial resolution: under appropriate irradiation conditions the material can be deposited in the form of single microdroplets, 9 which can have diameters as small as a few microns. 10 Moreover, LIFT can be used for printing different complex materials, such as inorganic inks or pastes, 11 organic polymers, 12 and even biological solutions.…”

“…18,19 Such interesting features prompted several studies on the transfer process which takes place during the LIFT of liquids. These studies 9,10,14,15,20 were mainly focused on the analysis of the effects that different technological parameters have on the morphology of the transferred material. This allowed improving the performance of the technique, although it also revealed the need for the elucidation of the transfer mechanisms in order to set the basis for parameters optimization.…”

Laser-induced forward transfer of pure metals

“…DRL-LIFT is a highly flexible transfer process, which restrictions are mainly determined by the adaption properties of the involved materials. So far, various materials [65,66] including polymers [67] as well as liquids [68] and living cells [69,70] have been successful transferred. A drawback DRL-LIFT is given by the residual components of the DRL, that may contaminate the deposits on the receiver.…”