On Ejecting Colloids Against Capillarity from Sub‐micrometer Openings: On‐Demand Dielectrophoretic Nanoprinting

Abstract

The printing of liquids and colloids has a large range of applications and often relies on the ability to eject in a controlled manner very small amounts of liquid from the opening of a capillary. To this end, in the conventional on-demand microprinting methods (such as ink-jet piezoelectric or bubble jet) since the liquid wets the inner wall of the capillary, the corresponding capillary pressure at the opening needs to be overcome and the meniscus accordingly deformed to the point where microdroplets are generated and ejected. However, this is prohibitively diffi cult in the sub-micrometer range, due to the large resisting capillary pressure at the ejection opening. This pressure is in the range of O(10 bar) for typical colloidal solvents for an orifi ce diameter of O(100 nm). Ink-jet printing of colloids for 2D and 3D structure generation, [ 1–3 ] on fl exible or infl exible substrates, [ 4 , 5 ] is, among other applications, an alternative approach to mask-based photolithographic microfabrication, but as stated above, also remains a challenge due to the high ejection pressure required for ultrasmall nozzle openings. Printing by electrohydrodynamic jetting profi ting from Taylor-cone [ 6 ] concentrated jet or serial droplet ejection [ 7 ] is a viable option for micrometer size pattern generation of conductive [ 8–10 ] and even biological materials. [ 11 ] The caveat is that this valuable technology relies on ionic liquids and/or charged particles [ 12 ] in the printing suspension, with possible unwanted side effects due to the effective charge deposition on surfaces and that the generated droplets at the tip of the Taylor-cone are not easy to precisely control. While uncharged particles of a colloidal suspension in nonpolar liquid environment cannot be manipulated by a homogeneous electrostatic fi eld, an inhomogeneous fi eld exerts a force FDE P ∝ ∇ |E | on a particle if its polarizability exceeds the polarizability of the surrounding carrier liquid. This is the effect of dielectrophoresis (DEP) [ 13 ] and has been employed for single particle manipulation in electrolyte liquids such as carbon nanotube placement, [ 14 ] separation, [ 15 ] and particle alignment [ 16–19 ]