An integrated micropump and electrospray emitter system based on porous silica monoliths

Abstract

The work presents the design of an integrated system consisting of a high-pressure electroosmotic (EO) micropump and a microporous monolithic emitter, which together generate a stable and robust electrospray. Both the micropump and electrospray emitter are fabricated using a sol-gel process. Upon application of an electric potential of sufficient amplitude (>2 kV), the pump delivers fluids with an electroosmotically induced high pressure (>1 atm). The same potential is also harnessed to electrostatically generate a stable electrospray at the porous emitter. Electrokinetic coupling between pump and spray produces spray features different from sprays pressurized by independent mechanical pumps. Four typical spray modes, each with different drop sizes and charge-to-mass ratios, are observed and have been characterized. Since the monolith is silica-based, this integrated device can be used for a variety of fluids, especially organic solvents, without the swelling and shrinking problems that are commonly encountered for polymer monoliths. The maximum pressure generated by a 100 microm id monolithic pump is 3 atm at an applied voltage of 5 kV. The flow rate can be adjusted in the range of 100 nL/min to 1 microL/min by changing the voltage. For a given applied voltage across the pump and emitter system, it is seen that there exists one unique flow rate for which flow balance is achieved between the delivery of liquid to the emitter by the pump and the liquid ejection from the emitter. Under such a condition, a stable Taylor cone is obtained. The principles that lead to these results are also discussed.

Understanding electrokinetics at the nanoscale: A perspective

“…As such, nanopores generate a high Maxwell pressure per unit volume when an electric field is applied, thus allowing the realization of high pressure ͑Ͼ5 atm͒ microcapillary electrokinetic pump. 2 Using streaming potential to generate electricity is also best done with a nanoporous membrane 6 for precisely the same reason.…”

“…Within our group, we have used nanoporous membranes as microfilters for removing debris or concentrating analytes, 1 as substrates for electrokinetic pumps and electrospray emitters into mass spectrometers, 2 and as effective chromatograph packing. 3 The membranes typically have a high surface charge density of about one charge per 10 nm 2 , and when their pore dimension is smaller than the electric Debye layer ͑EDL͒ thickness ͑i.e., there exists a strong EDL overlap͒, these membranes become ion-selective such that mostly counterions can enter and exit the membrane.…”

A low-voltage nano-porous electroosmotic pump

“…One of the most promising micropumps for LOC applications is electroosmotic (EO) micropump, which drives the fluid motion using the surface charge of a channel wall that can spontaneously develop when the channel wall is in contact with an aqueous solution [16][17][18], or that can be artificially created through the control of gate potentials applied to the gate electrodes embedded beneath the channel wall [14,19]. The EO micropumps provide a high-pressure head and adjustable flow rate and also have the advantages of flexibility and biocompatibility for various LOC applications, which include microelectronics cooling [20,21], high performance liquid chromatography separations [22,23], drug delivery [24][25][26], water management in fuel cells [27,28], and micro-injection system [29][30][31][32]. However, most currently available EO micropumps require a very high driving voltage (on the order of 0.1-1 kV) to generate a sufficient flow rate, which in turn leads to electrolysis of water, oxidation of electrode surface, and Joule heating; eventually limiting the use of EO micropumps in portable LOC devices due to the requirement of a high-voltage power supply accessory.…”

Producing of Monolithic Layers of Silica for Thin-Layer Chromatography by Sol-Gel Synthesis

“…The methods of monolithic layers of silica synthesis by acid and base hydrolysis of precursors described in literature [9,20], were tested and modified. The basic aspects of methods, changes that were made and results are presented below.…”