We planned to travel by Eurostar from Amsterdam to London on Sunday May 26 of this year; however, Eurostar tickets for that date are still unavailable online even though they are available for days before and after May 26. Therefore, we booked passage on the Eurostar train which travels direct from Rotterdam to London on May 26.
My question is about getting from Schiphol Airport via train to Rotterdam Central. I see on the NS International site that there are direct IC trains that make the journey to Rotterdam in 26 minutes. Is it necessary to purchase tickets in advance, or can we just show up on the day and purchase them? Is this done through a ticket window or machine? The NS International site makes reference to use of an OV chip card. Since we're US citizens, do we need this card or can we use our credit card with chip, like in London? Thank you for your insight.

Not necessary. Tickets are good on any train from A to B. Seats are not reserved. You scan or tap your ticket or phone to enter the station at Schipol, then again when you exit at Rotterdam. You can buy at machines which are plentiful and easy to use, or wiith the NL rr app on your phone. The chip card is a 1 euro add on ( or maybe less) when you buy a ticket. You can avoid that fee by paying on your phone. You may also be able to pay just by tapping a cc or apple/google pay on your phone. Im not sure. I use my phone or card to tap to pay all overcthe NL, even for purchases as small as 1 euro at say a food truck or street vendor. You may go your entire stay without ever needing to use cash.

Beware that groups of pickpockets may be on board trains so be sure to watch and stay with your belongings / bags.

Did you look for the regular train from Amsterdam to Brussels and Eurostar from Brussels to London?

But -- May 26 is the middle of the UK's Bank Holiday weekend so it is possible they are sold out.just

Thank you both for your input. Janisj, I did consider an IC train to Brussels and would have been able to book the Eurostar from Brussels to London for May 26. If I researched it correctly, the direct connect train to Brussels was a 2 1/2 hour journey; followed by a 2 hour Eurostar trip from Brussels.
On the other hand, the IC direct train from Schiphol to Rotterdam Central was only a 26 minute journey, and then the 9:28 Eurostar from Rotterdam to London is a direct train, no changes. So the Rotterdam choice seemed like a better one. I hope I did my research correctly! Thanks for your advice.

Easier to take the direct Eurostar to London from Rotterdam. If you take the IC to Brussel you'll have to go through check-in and immigration in Brussel, and if your train is delayed you may have a problem.

Put this in the wrong place!

see my long response on your thread . . .